Some material presented below overlaps the main article to present a standalone commentary.

Introduction
In the main article published in Micscape March 2015, the history of the use of wing scales from the Morpho menelaus butterfly as optical microscope test objects was presented in the context of modern studies of the genus. As remarked, the wing scales of this species had a key role in the optical development of the microscope. It was one of the oldest (if not the first) 'test object' to be specifically described by species in English by Dr. Goring in his 1827 paper (1). In a fascinating historical twist, the genus has re-attracted intense scrutiny under the microscope in the field of photonics to study the ultrastructure of some Morpho species' wing scales, which is only revealed under the electron microscope. 

While compiling this study with the essential contributions from Howard Lynk and Brian Stevenson, what appears to be an historical oddity surfaced which we haven't seen reported before. This is presented here separately from the main article in the hope that a reader may be able to offer an insight into the queries raised.

Essentially, prepared slides of the scale strews labelled 'Menelaus' or 'Morpho menelaus' inspected, dated ca. 1830s - ca. 1860, do match the contemporary descriptions and illustrations of the test object scales but do not match those of the modern M. menelaus. However, slides of sections of the upper wing prepared at the same time by the same maker do have the very different scales typical of the modern M. menelaus. This suggests that this mismatch was something more than an early 19th century understanding of the Morpho genus differing from our current classification. (Another famous test object, the scales of the springtail 'Podura' identified at a similar date to 'Menelaus' were the subject of taxonomic uncertainties for some decades as to which species was being referred to, see the Micscape article by David Walker.)

Illustrations of the test scales described as those of 'Menelaus'  and later 'Morpho menelaus'
The detailed illustration of the test scales by leading experienced workers, from Goring's first reports in the late 1820's onwards, are all consistent on how they illustrate the test scales described initially as 'Menelaus' and later as Morpho menelaus and typical examples are shown below. The early achromatic optics of the time were being assessed for their ability to resolve the main longitudinal ridges on the scales and their cross ribbing. They are shown as broad scales with expanding edges and barely toothed.

Early to mid 19th century slides showing the test scales as described by the workers at the time
A fellow microscopy enthusiast, Howard Lynk in the USA, is fortunate to own examples of the rare slides made at the time of the early test studies. This forms part of his collection of old Victorian microscope slides which are splendidly presented on his website with his own studies. A pair of ca. 1830s slides made by the same worker of a scale strew and part of wing of 'Menelaus' are shown below. It was common at that time for the same worker to prepare both of butterfly subjects.

The 'Scales of Menelaus' or later 'Scales of Morpho menelaus' slides below dating from the 1830s to ca 1860s clearly shows scales that match the descriptions and illustrations of the time when described as a test object. The scales have distinctive non-parallel expanding sides with barely any toothing and are partly translucent to permit ridge structure studies in transmitted light.

One puzzle immediately arises—the test scale morphologies shown below are very different to those from the upper wing of the species now regarded as Morpho menelaus which have iridescent narrow parallel sided scales (the ground scales) underlying the slimmer translucent cover scales (presented later below).

A later slide ca. 1840s shown below also shows a scale strew with the same scale morphology (with likely some toothed underwing scales as well).

A later slide ca. 1850s-60s by Norman shown below also shows a scale strew with the same morphology as those reported for the test scales (with likely some more opaque and toothed underwing scales as well).

Early 19th century slides showing sections of the upper wing of 'Menelaus' or 'Morpho menelaus'
It was common for a given maker to make both a scale strew slide and one of a section of the upper wing at the same time. The believed Cary-Gould and Bond pairings above are of this type. As the 19thC scale strew morphologies don't match those of the modern M. menelaus, what do the scales on the 19thC wing sections scales show?

The present authors are not experts on the Morpho genera—the taxonomy of the genus and that of M. menelaus in the 1820s could be different from today's but the scales in the earlier slides of parts of the upper wings do seem to match that of the modern species. Although there are many sub-species reported (see Wikipedia entry).

Modern compound microscope studies of M. menelaus ground or cover scales
Scales taken from a modern specimen of the adult M. menelaus are shown below using various sampling and lighting techniques. These were deferred from the main article.

Comments and queries
One potential explanation of the scales on the strews not matching those of the upper wing sections, we believe can be ruled out, i.e that the preparers were seeking a particular morphology of wider scale on M. menelaus for the test subjects. There is no evidence from my own or of Howard Lynk's careful studies of the whole upper wings under stereo microscopes of multiple modern specimens that these type of test object scales are present on M. menelaus. Neither do the published modern research reports on this species seen to date show any.

Early reports stated it was identified as a potential test object in the 1820s by Thomas Carpenter, London who was described as an 'experienced entomologist' and wrote extensively on entomological subjects for Gill's 'Technological Repository of the 1820s and 1830s.

Further support that they were not seeking out special scales on samples of M. menelaus is the early descriptions of where the scales were sourced on the butterfly—see extracts below from the work of the leading workers Andrew Pritchard and John Quekett.

Summary of scale morphologies from historical and modern sources
The table below compares the scale parameters of historically reported 'test' scales and modern observations. In my own work in the table, photomicrographs of a range of scales were printed out on A4 paper together with a micrometer scale. Ridges were measured over at least 10 ridges and averaged to give a value for the separation.

As the table attempts to summarise, there seems a baffling contradiction between the parameters of the original test scales (transparency, ridge separation, length : width ratio) and those for both cover and ground scales sourced from a modern M. menelaus. The transparency of a cover scale and the observation by Goring to select pale blue scales suggests that cover scales type scales rather than dark ground scales were the test object. But the modern cover scales have a very different shape to those of the 'test', the ridges are rather coarse and there's no cross striae described by the early workers.

Modern ground scales also don't match the 'test' scales. Although Goring remarked to avoid the dark scales when selecting test scales. Even assuming that their ground scales were more transparent than for modern examples, the ridges are much finer and likely pushing the capabilities of the early achromatic optics.

Access to an 1830s slide of the true test scale strew allows modern measurements (by Howard Lynk) to be compared with Goring's. The modern average ridge separation of 1.9 µm is in quite good agreement with Goring's value of 2.1 µm considering the limitations of the microscopes and the likely extra difficulties of micrometric measurements of the time.

Above. Image and extracted detail used to measure the scale ridge separation and scale sizes of the ca. 1830s M. menelaus strew
which has the true 'test' scale morphology. There are typically 13 -14 ridge spaces to 1/1000th inch. Image by Howard Lynk.

Queries arising:

1) What species of butterfly was used for the test scale strews—although Howard's slide strews match the contemporary reports of the scales as test objects, they do not match those of the modern M. menelaus?

2) For the slide pairings of scale strews and wing sections made at the same time by the same maker, why do the scales on the wing section not match those of the scale strews if the same source of butterfly was used? Although the wing sections do match those of the typical modern M. menelaus suggesting that they had not misidentified the species or that at the time M. menelaus referred to a different species. Do any of the many sub-species show this scale form?

3) Does the understanding of the Morpho genera and that of M. menelaus of the time offer any insight.

A possible contender for the true identity of the test scales is that of the species' upper wing shown right—an image courtesy of Professor H. Frederick Nijhout. Although ascribed in its caption to M. menelaus, in helpful email correspondence, it was likely that it had likely been incorrectly attributed but its actual provenance was uncertain. Although as noted in the table above, the ridges seem to fine to be the 'test' scales.

Image credit right. H. Frederick Nijhout Ph. D., Duke University. (Resized from master on www.nisenet.org)

The scales show similar features of the test scales—expanding sides and more translucent thus aiding the study and resolution of ridges and cross ribbing under brightfield illumination using the optical microscope of the late 1820s onwards. Such overlapping scales seem to preclude the possibility of cover scales.

Is any reader able to identify this species?

One further oddity which may be relevant is that from studying the slides owned by myself, Howard Lynk and Brian Stevenson there was a trend in the later 19th century for slides labelled Morpho menelaus to be of the lower wing scales only, i.e. the dense distinctly toothed scales, although this may have had an economic reason (see main article).

The authors would be very interested to hear from readers who have scale strews of the Morpho genus that match the test scale morphology.

Given the many years that have passed it may not be possible to resolve the puzzle but for the microscopist interested in the history of test objects, it certainly seems an intriguing one.

The author David Walker and colleague Howard Lynk are microscopy enthusiasts and don't have expertise in the Morpho or other genera and we welcome any comments / corrections.

Acknowledgements:
Thank you to Howard Lynk for photographing the early rare slides in his collection, for the splendid photomicrography and for providing information on the slide provenance. His imagery and knowledge of early slides was vital to help reveal the historical puzzle in combination with my own researches.
Thank you to Brian Stevenson for sharing images of slides in the main article.  
Thank you to Professor H. Frederick Nijhout, Duke University for permission to show the www.nisenet.org hosted image and for helpful discussions.

Any errors in the article are solely that by one of the authors, David Walker.

Howard Lynk maintains the splendid resource 'A Cabinet of Curiosities' at www.victorianmicroscopeslides.com.
Brain Stevenson maintains the splendid resource 'Historical makers of microscopes and microscope slides' at microscopist.net.

References: Many of the older journals and books are available on www.archive.org.

1. C R Goring, 'On Mr. Tulley's thick Aplanatic Object Glasses, for Diverging Rays; with an Account of a few Microscopic Test Objects', Quarterly Journal of Science, Literature and the Arts, 1827, XXII, 265-284. (Link is to Google Books beginning of paper.)

2. C R Goring, 'On Achromatic Microscopes, with a description of certain Objects for trying their Defining and Penetrating Power', Quarterly Journal of Science, Literature and Art, 1827, 23, 410-434. (Link is to Google Books beginning of paper.)

3. S. Bradbury, 'The Evolution of the Microscope', Pergamon Press, 1967. This book provides a detailed but very accessible discussion; chapter 5, 'The Development of the Achromatic Microscope'.

4. G. L'E. Turner, 'The Microscope as a Technical Frontier in Science', in ''Historical Aspects of Microscopy, Eds. S. Bradbury and G. L'E. Turner, The Royal Microscopical Society, 1962, pp.175-199. Figs. 9-11 plot the increase of numerical aperture and resolution of test objects with year showing the particularly rapid improvement from 1830s to ca. 1860.

5. A Pritchard and C R Goring, 'The Microscopic Cabinet ...', 1832, London.

6. J Quekett, 'Practical Treatise on the Use of the Microscope', 1848, London.

Note on the butterfly specimens. All recent specimens were sourced from an accredited European Lepidoptera dealer, imported under EU regulations from captive breeding programmes in S. America, run by and for the local people.

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