ZOOLOGY AND BOTA.NY, MICROSCOPY, ETC. 301 



a tank when it is desired to observe the crystal immersed in a liquid. 

 There are some other supplementary fittings. 



When the instrument is being used as a total reflectometer, and fairly 

 large crystal plates are available, the Microscope can, by means of a 

 special objective and ocular, be converted into a telescope. 



C4) Photomicrography. 



Colour Screens.* — J. E. Barnard has used the following methods in 

 the preparation of colour screens for photomicrographic use. Permanent 

 screens may be made by staining either gelatin or collodion films. A 

 1\ p.c. solution of gelatin in distilled water is melted down over a 

 water-bath and then filtered through glass or cotton wool. The indi- 

 cated dye is then added to the solution, and a suitable proportion of 

 gelatin then poured on to the plate and allowed to dry in a perfectly 

 horizontal position where dust is not likely to settle on it. Another, 

 and perhaps more preferable, method is to immerse a gelatin plate in a 

 solution of the dye until a sufficient depth of colour has been taken up 

 by the film. The solution of aniline dyes used should be of a strength 

 of 1 in 1000, with the exception of yellow dyes (e.g. acradine yellow, 

 auramine, or tartrazine), which may be used as strong as 1 in 250. 

 Enamel collodion may be used instead of gelatin, the dye being in this 

 case dissolved in absolute alcohol and added to the collodion before the 

 plate is coated. The best quality of enamel collodion must be used in 

 order to insure a perfectly clear film. 



The use of a fluid screen has the advantage of simplicity and the 

 easy control of absorbing power. For use with such fluid filters a simple 

 type of cell may be formed of a rubber band, from which a segment has 

 been cut, fixed between two thin glass plates by a pair of spring clips, 

 the depth of the cells being varied by the employment of bands of 

 different thicknesses. The dyes used are kept in stock solution, and a 

 fluid screen can be improvised at any time in a few moments. 



The use of a mercury vapour lamp as a source of light is of particular 

 advantage rn dealing with objects that are faintly stained with methylene 

 blue, and which often give only very faint images in photomicrographic 

 work. By using a screen that transmits the yellow mercury vapour 

 line, one can be perfectly sure that no blue is transmitted, and thus the 

 utmost contrast is secured. 



Directions are given for the preparation of the various fluid filters 

 that have given good results in the lecturer's hands : — Gifford's screen 

 (methyl or malachite green dissolved in warm glycerin), Zettnow's green 

 filter (cupric nitrate and chromic acid dissolved in water), yellow screen 

 (lo p.c. solution of acradine yellow in absolute alcohol ; one part beinu 

 added to four parts of enamel collodion), and blue screen (ammonio- 

 sulphate of copper). 



* Journ. Photomicrograph. Soc, 1915, iv. pp. 1-8. 



June 16th, 1915 v 



