ii2 ELEMENTARY TECHNOLOGY. 



other, large and adjustable for oblique light. Some turn over 

 the stage to illuminate opaque objects instead of a condensing 

 lens. 



8. Sub-stage is for carrying the Abbe condenser or other 

 illuminating apparatus, the polariscope, etc. It should always 

 have centering screws, and adjustment for focus is desirable. 

 In purchasing a microscope be careful to avoid disappoint- 

 ment. The advice of an expert is of advantage. Good work 

 may be done with patience by inferior instruments but micros- 

 copy in the future will depend upon the best glasses and 

 stands. For economy get such a stand as can be added to by 

 degrees. A I inch or ^ objective and a ^ or 1-5, will suffice 

 for general histological work. For high-power work, like 

 blood structure, bacteriology, etc., a homogeneous immersion 

 ^ or I -20, or an apochromatic of like power with an Abbe 

 condenser, or, better still, with achromatic condenser of wide 

 angle, will be needed. Whatever form of stand is procured, 

 have it of hard brass, with heavy foot, well balanced, no back- 

 lash in racks or screws, inclinable, with plane and concave 

 mirrors, with the Society screw for objectives, etc. This latter 

 is quite important. 

 4. Binocular instruments. 



These give a stereoscopic view of objects and are used for the 

 display of fine pictures under low powers. They divide the 

 rays by using a Wenham prism in a small box sliding over 

 the objective. Bausch and Lomb have contrived a swinging 

 prism. The body tubes are made adjustable for different eyes. 

 For high powers binocular eyepieces are made by Zeiss & Tol- 

 les, which act well. 



III. PREPARING OBJECTS FOR THE MICROSCOPE. 



i. Living tissues. 



Examine in natural fluids or indifferent or neutral fluids, as 

 serum, aqueous humor, amniotic fluid, or dilute albumen. 

 (Dissolve 2 grammes dried sodium chloride in 25 C C of 

 water, add 28 grammes egg albumen, 2.3 C C tinct. iodine and 

 2 or 3 drops carbolic acid. Mix thoroughly and filter. This 

 is termed iod-albumen.) The moist chamber, the warm stage, 



