L899] MICROSCOPICAL JOURNAL 183 



gold-size at intervals of twenty-four hours, and finally a 

 thin coat of Ward's brown cement. 



Oil of Cassin. — The high refractive index of oil of 

 cassin, and the fact that it dries hard enough to make 

 permanent mounts, renders this reagent particularly use- 

 ful to microscopists. It has an index of 1.6016, and 

 clears from eighty per cent alcohol. 



Residue. — To determine the character of a residue, 

 place a small portion on a glass slip and saturate it with a 

 drop of water. Note the general characters of the par- 

 ticles with the aid of a one-inch or a half-inch objective. 

 At the end of the glass slip, place a drop of a ten per cent 

 solution of sulphuric acid, and at the opposite end a small 

 piece of blotting paper. Diffusion will take place, and if 

 any particles of carbonates be present, they will reveal 

 themselves by effervescing. Quartz grains, clay, and car- 

 bonaceous matter will remain unaffected. 



Colored Screens. — For the purpose either of increas- 

 ing the contrast between different colored objects in a 

 specimen, or of reducing it, there are useful adjuncts to 

 the equipment of the microscopist and the photo-micro- 

 grapher. To obtain perfection of definition and of con- 

 trast either for visual or micro-photographic work, the 

 use of partial monochromatic light is essential when 

 working on bacteria and similar subjects ; and the fol- 

 lowing plan for making reliable screens may, therefore, 

 be of assistance to those who are engaged on this branch 

 of study. It was first suggested by Mr. Wall to Dr. 

 Spitta, the author of "Photo-micrography." Coat a 

 patent plate with a two and a half per cent solution of 

 albumen ; and, when dry, pour over it one hundred and 

 seventy minims of an eight per cent solution of gelatine, 

 level and allow to dry. To make red, orange, green, or 

 violet screens, soak the prepared plates in a one per cent 

 solution of crysoidine, aurantia, napthal green, and 

 methyl violet respectively. For yellow screens, soak the 



