182 



KNOWLEDGE. 



Junk, 1915. 



MICROSCOPY. 



By F.R.M.S. 



THE QUE RETT i\IICROSCOPICAL CLUR— The five 

 hundred and seventh ordinary meeting of the Quckctt 

 Microscopical Chib was held at 20, Hanover Sq\iarc, W., 

 on Tuesday, April 27th. Mr. Ainslic, K.N., introduced 

 the following paper entitled " An Addition to an Objective." 



Few niicroscopists who have made much use of high- 

 power dry objectives have failed to realise the connection 

 between the tube-length and the thickness of the cover- 

 glass if gootl definition is to be obtained. This is, indeed, 

 mentioned in te.\t-books, but not, as a rule, at any very 

 great length. I'"or instance, little is said as to the amount of 

 alteration required in any given case. The sensitiveness 

 of objectives varies enormously : it varies with the formula 

 on which the objective is constructed, but more especially 

 with the power of the objective. As an example, half- 

 inches of high aperture, such as the Holos and Zeiss 

 apochromat, require only a change of one or two millimetres 

 in the tube-length to compensate for a variation of -01 

 millimetre in the thickness of the cover-glass ; for a one- 

 si.xth the figure is from nine to thirteen millimetres ; while 

 for a one-eighth, such as the Leitz No. 7, the figure is as much 

 as twenty or twenty-one millimetres. N\'ater-immersions 

 are also subject to this sensitiveness, though to a smaller 

 extent, the figure in the case of a Zeiss " G " being 9-2 

 millimetres. 



This difficulty is more serious than is generally realised, 

 and is enhanced by the extremely small range of draw- 

 tube in the average Continental stand and, unfortunately, 

 in many stands of English make. The present paper is an 

 attempt to find a way out of this difficulty, due to the range 

 of the draw-tube being in many cases insufficient for the 

 proper examination of specimens with cover-glasses of 

 abnormal thickness or thinness, especially with the higher 

 powers. 



Many years ago the celebrated Van Heurck used what 

 he called a " transformer " as a means of making a long- 

 tube objective work on a short tube, and vice versa. This 

 consisted of a convex or concave lens of low power, fitted 

 above the objective, which, it will be readily understood, 

 affords a means of altering the actual plane in which the 

 image is formed, without affecting the action of the objective, 

 should the cover be of such thickness as to require, for the 

 proper working of the objective, a tube-length which 

 would bring the image beyond the limits of the draw-tube. 



With the high-power dry objectives in most common use, 

 such as a one-sixth, the power of the additional lens required 

 to effect the compensation for a very great change of cover- 

 thickness is not great, a pair of lenses — convex and concave 

 — of about three diopters power, or about thirteen inches 

 focus, being sufficient to correct for a very considerable 

 range of cover-thickness ; but with higher powers, such 

 as eighths, the amount of correction which can be got in 

 this way is a good deal less, as might be expected from their 

 much greater sensitiveness. As an example of what can 

 be done with an objective of not too high power, it may be 

 said that a Watson one-sixth, of N. A. -74, which is normally 

 correct for a cover -18 millimetre thick, and a tube-length 

 of two hundred millimetres, can be made to work well 

 through a cover-glass as much as -5 millimetre thick, if 

 a concave lens of — 8 diopters be placed behind it ; while 

 with a convex lens of the same, or somewhat lower, power 

 it will work well on an uncovered object ; and many other 

 objectives of this power will do as well. 



I have so far experimented only wdth simple lenses, but 

 the chromatic and spherical corrections are not perceptibly 

 affected unless the power of the additional lens is as great 

 as ten diopters, and even then the effect is not serious, 

 and is not appreciable at the centre of the field. 



The power of the objectives is somewhat reduced by the 

 convex lens, as well as the N.A. ; w-hile with the concave 

 lens the effect is the opposite ; but the change is not great 

 if tiie additional lens be placed as near to the back lens of 



the objective as possible, though, for reasons of convenience, 

 it does very well to put it in the rear of the mount. 



If for the oil in which an oil-immersion objective is 

 immersed wc substitute water, the effect is the same in 

 kind (though greater in degree) as the effect of a thinner 

 cover-glass in the case of a dry objective ; and it is possible 

 to convert an oil-immersion into a very good water- 

 immersion by merely fitting behind it a convex lens of 

 suitable power. The power cannot be predicted, but must 

 be determined experimentally for each objective. Here, 

 again, it is easier to effect the conversion in the case of an 

 oil-immersion of moderate power, such as a one-tenth, 

 than in the case of a one-twelfth or higher power, though 

 a one-twelfth can be dealt with very satisfactorily. A 

 Watson " Parachromatic," for example, requires a convex 

 lens of ten diopters, and it is important in the case of oil- 

 immersions of this power to place the additional lens as 

 close as possible to the back lens of the objective, to avoid 

 too great reduction of the working distance, which is to 

 a certain extent unavoidable. 



With an oil-immersion thus converted into a water- 

 immersion, it is useless to expect that the whole aperture 

 will be available ; the corrections of the objective are too 

 much upset for that ; but, if the additional lens is made of 

 such diameter as to reduce the N.A. to about 1-1, and an 

 illuminating cone of not more than about -75 or -80 N.A. 

 used, the performance is, in all cases tried, quite up to the 

 standard of the ordinary water-immersion, and better than 

 some. The additional lens may very conveniently be fitted 

 to the " funnel stop " usually supplied with oil-immersions 

 for the purpose of reducing the aperture for dark-ground 

 illumination, taking the place of the stop. In this way it can 

 be brought close to the back lens, and the working distance 

 is not reduced more than is necessary and unavoidable. 



It is believed that this method of converting an oil- 

 immersion into a water-immersion has not been previously 

 described ; it is hoped that it may be of use to those who 

 occasionally require to use water-immersions in work on 

 living specimens, or in other work for which an oil-immersion 

 would be inconvenient. 



The Honorary Secretary, Mr. J. Burton, then read 

 " Notes on Diatom Structure," by Mr. A. A. C. Eliot 

 Merlin, F.R.M.S. He drew attention to a very beautiful 

 form of tertiary structure he recently found on a variety 

 of A ulacodiscus comberi from Oamaru. The valve is on a 

 styrax-type slide of two hundred and thirty forms from that 

 locality, and is covered with a network of dark, well- 

 defined secondaries, except on the parts occupied by the 

 large primaries. Each of the dark secondaries splits up 

 into three or four parts by a bright cross-bar arrangement. 

 This structure requires a good oil- immersion objective 

 and a very considerable magnification to render it readily 

 discernible ; but it is not a glimpse object, and when well 

 seen reminds one of the bridges of bright matter that are 

 frequently observable crossing the umbrae of sunspots. 



A photograph of the above was exhibited, and Mr. E. M. 

 Nelson, F.R.M.S., confirmed the presence of this structure 

 from a specimen in his cabinet. 



IMr. Merlin also exhibited two other photographs of a 

 diatom. Mr. Nelson had written to him that he had dis- 

 covered that Coscinodtscus simbirskii, which with ordinary 

 transmitted light resembled Coscinodiscus asteromphalus, 

 when examined with dark ground and a rather small stop 

 looks like Actinoptychiis splendens. This led him to search 

 for the diatom specified ; and, although this could not be 

 found, he found one which, with dark-ground illumination, 

 revealed a beautiful radiating structure, somewhat 

 resembling a HeUopelta, which was not observable by 

 transmitted light. On the photographs of this specimen 

 being examined, it was identified by Mr. Morland as 

 Janischia antigna Grunow. 



Mr. Merlin further pointed out that, although " diatom 

 dotting " has influenced the development of the microscope 

 towards perfection more than anything else, be is unable to 

 find out particulars of its introduction. 



