THE OBJECT 105 



}'r(>p;irMiion.s made only for study can he, i)u(, in Hit' \\^\\\, 

 Mild watched, often with Jidvaiit.'ige, in the stages of fading. 

 Doubtless a few of the best objects can be kept for con- 

 sultation. But it is sometimes easier to make a fresh 

 preparation, than to find a fadeless stain, or a neutral 

 mounting medium. Brazilin-stained specimens, however, 

 have kept for over a year in immersion cedar oil (Leitz) 

 without perceptible fading. 



A Classification of Objects. — Different classes of objects 

 are met with in scientific research, and in routine work, and 

 these need different methods. 



1. Sohd objects {i.e., objects in which all three dimen- 

 sions have to be considered) of fair size, which often require 

 dissection or other manipulation, usually under water. 

 For all such, the spectacle magnifiers with magnification 

 of 2 (Fig. 5), and the low-power twin-objective binocu- 

 lar (Fig. 11) with powers of about Sj^'^ or 4 to 10, are the 

 instruments required. The layer of water over the dissec- 

 tion, however, should be as thin as practicable, consistent 

 with complete immersion, for the distortion and injury 

 to definition increase with the thickness, as they do with 

 too thick a cover-glass on the standard microscope. The 

 central axis of the binocular magnifier should be kept at 

 right angles to the surface of the water. 



2. Solid objects of rather small size, often in water, and 

 requiring manipulation or dissection. For such work the 

 stereoscopic binocular with twin objectives (or perhaps, for 

 the higher magnifications, the monobjective erecting 

 binocular — Lihotzky or Swan-Porro), magnifying from 10 

 to 50, is required. For examination of small algae and 

 rather small animals, in fresh or salt water, undisturbed, 

 the water-immersion twin objectives made by several 

 firms are to be preferred. For all observations under water 

 with dry objectives, the layer of water should be as thin 

 as convenient; and so it is probably better for the vessel 

 to have a flat, rather than a concave bottom. A cover-glass 

 is not usually required, either for objects in water, or for 

 those in air (except occasionally to flatten the surface of a 



