440 



KNOWLEDGE & SCIENTIFIC NEWS. 



[May, igo6. 



Conducted by F. Shillington Scales, b.a., f.r.m.s. 



Elementary Photo-micrography. 



{Continued from page 416.) 

 I ii.w K now (l.-.ilt at some length with the camera and 

 the adjustment of tlie microscope, and it only remains 

 to add a few words on the photographic process itself. 

 I shall assume that the reader has some experience of 

 ordinary photography, and shall content myself, there- 

 fore, with explaining such modifications as are necessary 

 for photo-micrography. Formerly an initial difficulty 

 made itself felt owing to the difference in focus between 

 the \isual and actinic rays of an objective, by which I 

 mean that though an object might appear to be in focus 

 on the ground-glass or plate-glass screens when 

 examined visually, the photographic negative when de- 

 veloped was found to be out of focus owing to the fact 

 that the actinic rays were not identical with the visual 

 rays. Owing to the improvement in objectives, how- 

 ever, by which they tend towards apochromatism, and 

 to the use of orthochromatic plates and certain screens, 

 this difficulty seldom occurs now. Therefore possessors 

 of modern objectives will have little trouble on this 

 score, and such objectives may be used either with or 

 without an eye-piece, but in the former case a projection 

 ocular should be obtained, and will give better results. 

 Huyghenian oculars do not perform well in photo- 

 graphy. Apochromats cannot be used alone, but per- 

 form excellently with their compensating oculars as 

 well as with projection oculars. 



There comes in, however, rmother difficulty. Unless 

 an object has good contrast it is manifest that the re- 

 sulting photograph will be lacking also in contrast, 

 but what is less evident is that the stains with which 

 microscopical slides are stained may differentiate struc- 

 ture sharply enough to the eye, and yet, owing to the 

 varying actinicity of the colours, fail to differentiate 

 the same structure satisfactorily in the photograph. 

 An ordinary photographic plate, for instance, shows a 

 great susceptibility to the rays at the violet and blue 

 end of the spectrum, and very little to those at the 

 yellow and red end. 



If the object were merelv black and white this would 

 not be material, but supposing we were trying to photo- 

 graph a violet or blue object on a yellow ground we 

 should find that the object would impress itself upon 

 the plate as fast as the ground, and there would be no 

 contrast or differentiation. We should, therefore, need 

 to keep back the violet of the object and to assist the 

 yellow of the ground. Our first method of dealing with 

 this is to use an orthochromatic plate, namely, one 

 made either actually or comparatively more sensitive to 

 yellow, and our second methcxl is to use a screen, in this 

 case a piece of dense yellow glass, which will cut off, or, 

 at least, keep back, the violet rays, and so assist the 

 yellow rays to impress themselves upon the yellow 

 sensitive plate. Of course, the treatment will be varied 

 according to the staining, and it might even happen that 

 with a pale yellow object we might need to use an 

 ordinary plate and a blue or green screen, but enough 

 has Ijeen said to show the nature of the difficulties be- 



setting the photo-micrographer, and the way to deal 

 with them. Generally speaking, the principle is to 

 keep back the colour with which an object is stained 

 by using a plate which is not sensitive to that colour, 

 and a screen (generally of an opposite colour) which 

 tends to nullify it. 



(To be continued. ) 



Royal Microscopical Society. 



Al the meeting held on March 2i, the Rt. Hon. Sir 

 1(11(1 North, I'.C, F.R.S., Vice-President, in the chair, 

 the death of Mr. J. J. Vezey, Treasurer of the Society, 

 was announced. Mr. Wynne E. Baxter, J.I'., F.G.S., 

 was appointed Treasurer in succession to Mr. Vezey. 

 Mr. j. W. Gordon exhibited and described a new retro- 

 ocular or top-stop for obtaining dark ground illumina- 

 tion with high-power objectives, and increasing the 

 definition of highly-resolved images in a bright field. 

 Mr. C. F. Rousselet read a paper entitled " A Contribu- 

 tion to our Knowledge of the Rotifera of South 

 Africa," illustrated with lantern slides and mounted 

 specimens. Mr. J. M. Coon exhibited and described a 

 new lorm of finder, which could be used on any micro- 

 scope, and with high powers, and by which the object 

 registered on one microscope could be found on any 

 other. The Secretary read an abstract of a paper by 

 Nfr. X. D. F. Pearce " On some Oribatidse from 

 Sikkim." A paper by Mr. E. M. Nelson " On the 

 Limits of Resolving Power for the Microscope and 

 Telescope," being of a technical character, was taken 



as read. 



Quekett Microscopical Club. 



.\t the meeting held on March 10, Mi-. C. D. Soar, 

 F.R.M.S., gave a lecture on "The Life-History of 

 Fresh-water Mites (Hydrachnidee)." The life-history 

 may be divided into four stages — ovum, larva, nymph, 

 and adult. Descriptions were given of some of the 

 various methods adopted by different species in de- 

 positing the eggs. The incubation period is about 30 

 days. The dexelopment of the embryo w'as described 

 and illustrated by a series of figures drawn every two 

 days from the deposition of the egg until the emergence 

 of the larva. In the second stage of the life-history the 

 larva usually becomes parasitic, as a rule selecting a 

 host which lives in the water. The insects most com- 

 monly infested were mentioned, and several specimens 

 from the genus Dyiiscus were show^n in spirit with a 

 number of the larvse attached like small bags to the 

 ventral surface. D. marg'uialis was said to be a very 

 favourite host. After describing the nymph stage, the 

 lecturer showed by means of the lantern a number of 

 interesting drawings of adult forms of some of the 60 

 genera at present known. The life-history of these 

 mites was first described by Miiller in 1S34. 



Mobile Crystallisation. 



The Times, of January 3, quoting from the Frank- 

 furter Zcitung of December 12, has an interesting and 

 suggestive note on a new physical action which has 

 recently been observed in the Physical Institute of the 

 Karlsruhe Polytechnic, a fuller account of which was 

 to appear in the Annalen der Physik. It was observed 

 there some time ago that in certain kinds of solutions 

 tending to form emulsions, the existence of extremely 

 soft, almost liquid, crystals was possible, which, under 

 certain conditions, assumed the forms characteristic 

 of solid crystals, cubes, prisms, and the like. In other 

 cases, for instance, when under the influence of pro- 

 tracted cooling, the cohesion, and with it the surface 



