WITH THE MICROSCOPE. 259 



stop on its curved base (for obtaining oblique light in parallel 

 rays) when photographing opaque and semi-opaque objects, as 

 the Foraminifera, &c. Dr. Moitessier recommends this plan, the 

 light being transmitted from the mirror through the cell of ammonio- 

 sulphate of copper, then conveyed by the condensing lens on to 

 a disc of ground glass, placed near the apex of the cone. Dr. 

 Maddox has lately obtained stereoscopic pictures of parts of 

 Pleurosigma formosum, magnified 3,000 diameters, with Mr. Wales' 

 -|-th objective and amplifier. Seep. 231. 



CHEMICAL SOLUTIONS REQUIRED. 



The different solutions used in photography must be perfectly 

 pure ; this is of the first importance, and observers are recom- 

 mended to purchase their chemicals at houses of known celebrity, 

 such as Mr. Thomas', Pall Mall, rather than attempt the manufacture. 



S3 1 ?, collodion. Supposing the collodion process to be deter- 

 mined on, the pyroxyline should be of the kind produced from 

 hot acids, carrying just such an amount of water as will furnish 

 to it when dissolved in its solvents, ether and alcohol, a fluid 

 flowing freely, possessing considerable adhesive power to the glass, 

 and free from fine net-like markings when dry. The manufacture 

 of the gun cotton that will furnish these qualities requires great 

 experience. The collodion should afford when taken from the 

 nitrate bath, not a very thick creamy layer, but such as is com- 

 monly employed for portrait purposes. If it be preferred to make 

 the collodion, we subjoin the formula for cotton that will yield the 

 above mentioned film. Into a perfectly clean dry close stoppered 

 bottle, put 



Iodide of ammonium in crystals." 

 Iodide of cadmium, of each, 8 grains. 

 Bromide of cadmium, 4 grains. 



Pour on these 13 drachms of absolute alcohol or redrawn 

 alcohol, of sp. gr. '805, shake the bottle well ; when dissolved, 

 add- 



Pure ether, sp. gr. 725, 12 drachms. 



Weigh out 22 to 28 grains of dry pyroxyline, add it by little open 

 tufts to the mixed fluid, shaking occasionally, then wash down the 

 neck and sides of the bottle with 8 drachms of pure ether. Gently 



* If the crystals of iodide of ammonium be at all clamp, press them before 

 weighing in folds of clean blotting paper. 



S 2 



