Junk, 1912. 



KNOWLEDGE. 



239 



objective, which ought to have been provided with a smaller 

 internal diaphragm. 



.Any inicroscopist who will criticall 

 other small objects in various media 

 can confirm these statements. Air, 

 water, salt solution, glycerin in various 

 dilutions, chloral hydrate mi.\tures, 

 glycerin jelly, paraffin, dammar, 

 colophonium, bergamot oil or turpen- 

 tine dilutions of these last two, serve 

 to exemplify media of low refractive 

 index; of these I have found paraffin 

 (as reconnnended by Dr. Coles. 

 " Knowi.edgk," Vol. XXXIV, page 

 192) to be by far the best, and glycerin 

 jelly by far the worst, on account of 

 its instability, the action which (as an 

 aqueous medium) it has upon the 

 glass, its want of transparency, and 

 the dilTiculty with which it is removed 

 from the object when it is desired to 

 remount. Of media possessing higher 

 indices than balsam, I have tried 

 styra.x, styrax in monobromide of 

 naphthalin, the monobromide alone, 

 and methylene diiodide. These are 

 optically better, on the ground that 

 they give better definition in depth, 

 which is often greatly desired in 

 photography : but for ordinary use they are ineligible, the 

 liest being the monobromide styrax. which, however, has 

 the grave disadvantage of depositing gum after a time, besides 

 undergoing a change not yet sufficiently 

 investigated. I recommend that objects 

 to be viewed by contrast shall be well 

 dehydrated and brought into paraffin. 

 If no more than the requisite amount 

 of paraffin be used, there is no need to 

 secure the cover. Paraffinum liquidum, 

 not fluorescent, can be obtained of any 

 chemist. If remounting is necessary, it 

 is perfectly easy to wash the preparation 

 in xylol, when it may be treated in any 

 of the usual ways. Mounting in paraffin 

 is thus extremely simple and good as a 

 temporary method. The liquid pene- 

 trates a dehydrated specimen with great 

 ease, and remains unchanged. To com- 

 plete the dehydration of larger specimens 

 it is well to clear with creosote or clove 

 oil. Paraffin specimens are best for 

 photographic purposes. 



In accordaiuo with the considerations 

 adduced abo\e, the definitive method 

 will consist in mounting the specimen, 

 after staining, in Canada balsam. Now, 

 therefore, it only remains to describe 

 the staining. .\ radula boiled out in 

 caustic alkali will be already fixed to 

 some extent. Almost any stain will now 

 colour the newly-formed end of the 

 organ. The rest of the structure will 

 not stain regularly with any reagent. 

 It will do so, however, and in. a definite 

 way, after a second fixation, or mor- 

 danting. .Any of the usual histological 

 fixatives may be used, or plain solutions 

 of potassium bichromate, permanganate, 

 or iodine. Tincture of iodine gives 

 very good results, and should be applied for such a time that 

 after washing out with absolute alcohol the structure is still 

 distinctly stained yellow. Stain next with Biebrich Scarlet 

 until the basal-plates throughout are strongly coloured. 

 Remove superfluous stain with cigarette paper, and wash in 

 absolute alcohol until no more stain comes away; a drop or 

 two will suffice for this purpose. The newly-formed unci 



examine radulae or again with distilled 



FlGTRK iTiT. 



A photo-micrograph of a living Crustacean 



[Cyclops) taken wuth the camera seen in 



Figure 268. Exposure half-a-second. 



Figure 268. 



.A new instantaneous photo-micrographic 



Camera. 



will now be seen coloured a brilliant red, and the basal-plates 

 throughout with a duller red ; this colour is fast. Wash 

 water, and stain for some time in an 

 aqueous solution of thionin ; this will 

 not affect the parts already stained, 

 but will clearly colour the points or 

 pectinated parts of the unci. The 

 preparation so made shows all the 

 detail that is desired ; there is no 

 uncertainty about any part of it, and 

 optical illusions are reduced to a 

 minimum. You may focus through 

 the specimen and thus optically 

 dissect it. 



It is perhaps desirable that some 

 other stain should take the place of 

 thionin. which might be fugitive in 

 Canada balsam. But in this case, 

 perhaps, the thionin has formed a 

 compound with the iodine ; it is cer- 

 tainly dislodged less easily than from 

 sections. In any case, it is probable 

 that a substitute of undoubted per- 

 manence could be found. Mordanted 

 specimens may also be stained with 

 Kernschwar^, when they can be 

 photographed. A balsam specimen 

 stained ten years ago in gentian violet 

 is still as good as ever : but it will be 

 noticed that the particular object of the Biebrich .scarlet and 

 thionin succession is to provide a selective staining of different 

 elements. For this it is necessary that the first stain should 

 colour only new-formed unci and basal- 

 plates, and that the second stain should 

 form no compound colour with the first, 

 nor wash it out. It is plainly impossible 

 to represent the contrasting colours by 

 simple photographic methods, but they 

 serve most materially to elucidate the 

 structure. Radulae fixed in bichromate 

 m ly be stained yellow with osmic acid ; 

 this yellow colour may be " developed " 

 to black by means of pyrogallol. This 

 method, originally suggested by Bolles 

 Lee in 1887, gives most excellent results 

 with radulae which show but small 

 basal-plates. It is probably the best 

 method for Pectinibranchs. 



Radulae can be fixed to the glass of 

 the slide by covering them with a drop 

 of bichromate when they have been 

 arranged on the slide, and allowing it 

 to soak well into them (two or three 

 minutes). This must be done by dim 

 or red light. Now cover and expose the 

 back of the slide to sunlight, so as to 

 cause the formation of the insoluble 

 chrome compound in the membrane. 

 Drain off the superfluous liquid and 

 leave to dry. On subsequent washing 

 the specimens should be found well 

 fixed, in both senses. The cover used 

 in this process may be another slide 

 S tied on the first, and a thin film of 

 paraffin on its inner surface prevents 

 the front of the radula from sticking 

 to the cover. 



The image-erecting dissecting micro- 

 scope with Porro prisms is of the greatest 

 value for dissecting out radulae and in all mounting processes. 



E. W. BOWELL. 



A NEW INSTANTANEOUS PHOTO-MICROGRAPHIC 

 CAMERA. — At the meeting of the Royal Microscopical 

 Society, held on April 17th, Mr. F. Watson Baker exhibited 

 a new reflex photo-micrographic camera for the instantaneous 



