November, 1910. 



KNOWLEDGE. 



443 



This is due to several causes. 

 2nt!. the method of illuminating; 



1- 



PODUKA SCALES. 

 To the Editors of " Knowledge." 



Sirs, — From the ver\able article upon the Podura Scale by 

 Mr. T. L. Smith in your 

 September issue, arises 

 Phoenix-like that very old 

 standing enigma to micro- 

 scopists,'"What is its actual 

 structure and wherein lie 

 the difficulties of observa- 

 tion, seeing that the object 

 is coniparati\ely large ? '" 



When we consider the 

 instruments in use upon 

 this scale as far back as 

 1827. the year about which 

 it loomed forth as a "test," 

 and the very much ini- 

 pro\ed optical appliances 

 of the present day. it would 

 seem that its accurate 

 delineation has not ad- 

 vanced in progressive ratio. 

 1st, the method of mounting: 

 3rd. the extreme fineness of 

 the scales usually selected; 

 4th, the necessity for 

 correct adjustment in tube 

 length for cover thick- 

 ness ; and 5th, the scarcity 

 of fresh scales. 



With regard to the 

 mounting, as the refrac- 

 tive index of the scale 

 itself is high. To at least, 

 to momit it in Balsam 

 is to practically obliterate 

 it, while Realgar or 

 other high refractive 



mountants do not appear to have been used with success, 

 so that we are left with the only alternative in contrast 

 between the normal of air and the scale 

 itself ; here, then, we make no advance upon 

 the ver\- earliest methods. True, we augment 

 ver>- much our view by placing the scale flat 

 upon the under side of cover glass, leaving a 

 small margin for air between it and the slip, 

 but therein lies the difficulty. As the object 

 is not perfectly even, it is almost impossible 

 to obtain optical contact aU over, so that 

 where the scale leaves the coverglass ever 

 so slightl_\' we get a distorted appearance, and 

 some of the views thus obtained account in 

 no small degree for conclusions as to new 

 structure being obser\ ed. 



As to the manner of illuminating, it is usual 

 nowadays to holdfast to the so-called "Critical 

 Method," which means practically placing the 

 scale in the midst of your illuminant and 

 viewing both focussed in the same plane, at 

 the same time having them centred axially. 

 When this is done with the Podura there is 

 insufficient contrast to bring out several of the 

 finer phases of detail. It is much the same as 

 if one placed a piece of plain glass in the 

 midst of a lamp flame and were asked to 

 observe its une%en surfaces, or to direct your 

 telescope boldly towards the full moon and 

 expect to trace its finest details. There are 

 many microscopists of the present day who 

 do not find in the practice of this '' Critical 

 Method," that which theor>- would have them 

 believe as ipso facto. 



I suggest therefore the use of oblique light 

 in this instance, and the photographs here reproduced have 

 been taken more or less in this wav. 



Figure 1, 



FiGf 



Podura 



-Again, the exceptionally fine scales generally experimented 

 with. I belie\e those usually procurable are taken from near 

 the head of the insect, but it is possible to obtain others much 



more decidedly marked 

 along the side and dorsal 

 portions. It has also been 

 suggested that one might 

 rear a few Podurae using 

 some innocuous aniline 

 stain along with their food, 

 such as Medical Methylene 

 Blue, with a view to obtain- 

 ing — possibly — through an 

 absorption of colour, a 

 scale showing greater 

 contrast. To this, how- 

 ever, I can only say my 

 own effVjrts have been 

 entirely abortive. 



I have found that small 

 well-defined scales gave 

 me the best results, whilst 

 some of the larger variety were so exceedingly fine in their 

 longitudinal as well as transverse markings as to appear mere 



shadows of themselves, 



Fourthly.it is ver\essen- 

 tial to haveawell-corrected 

 objective, both chromati- 

 cally and spherically, and 

 to carefully adjust the tube 

 length for thickness of 

 cover in use, 



.And lastly, the dearth 

 of fresh scales, A veiy- 

 eminent microscopist wrote 

 recentlythat Podura scales 

 were " scarcer than dia- 

 ;. _. monds," and it would seem 



there is much truth in that 

 remark at the present time; yet we are told the insects abound 

 plentifuUv, that some species are quite common about the 

 houses and .gardens, in damp cellars or vaults, 

 or again to be found among the dr\' refuse and 

 corners of outhouses, potting sheds, and the like. 

 If some expert entomologist would come to 

 the rescue and place a number of fresh scales 

 upon the market worthy of careful obser\a- 

 tion, he would not only relieve a much felt 

 want, but at the same time give a much needed 

 impetus to the improvement of condensers and 

 their proper usage for high-angled apochromats ; 

 both as regards their aplanatism and the 

 aperture suitable with particular objects, 



A word or two with regard to the photographs. 

 In Figures 1 and 2, it will be seen that apart 

 from the usual exclamation marks, there are 

 long sinuous lines from root to tip, while in 

 Figure 3 can be readily observed similar lines 

 transversely. 



If the exclamation marks be simply wedge 

 shaped pouches with an oily secretion within, 

 these longitudinal lines appear to run over the 

 outer edges of the wedge and again to dip under 

 beneath the preceding one, interlacing with 

 the transverse lines and forming an elastic 

 but strengthening structure to the whole scale. 

 It is noteworthy that either side has always 

 presented the same appearance to me. so much 

 so that I am unable to recognize any top or 

 bottom as distinct from one another. The piu"- 

 pose of these markings can only be conjecturalat 

 present ; it may be they are the edges of a flat- 

 tened cellular structure.thoughthiscan hardly ac- 

 count for the transverse ones.or simply a finenet- 

 work of tubes for the purpose ofconveving some form of nutrition, 



F, J, W, PLASKITT, 



