MICROSCOPE OUTFITS 159 



ail apochromatic water-immersion objective for much of 

 his chromosome woi'k on objects in acetocarmine. Coles 

 used, as ah-eady stated, as a finder, an apochromatic 8-milU- 

 meter objective, with dark field, on stained films mounted 

 dry. A celebrated authority on cells and chromosomes 

 used a 1.5-millimeter apochromatic objective for most of 

 his work of observation, drawing, and photographing. 

 For measuring and observing chromosomes, the writer 

 prefers (for several advantages) the 3-millimeter apochro- 

 matic oil objective 60 of 1.3 aperture, with an eyepiece 

 magnifying 20 times. It is evident that different workers 

 have different judgments. In fact, all modern objectives, 

 achromatic, fluorite, or apochromatic, are so good when 

 accurately centered that good work can be done with all; 

 and the real differences which exist are slight, and often 

 only noticed by the skilled worker. 



Moderating the Light. — All microscope outfits should 

 include means for moderating the light. The chief need 

 for much moderating of the light comes from the changing 

 of eyepieces. If eyepieces are kept so that the total magni- 

 fication is about 750 to 1,000 times the working aperture, 

 then the need for light moderating is confined to the change 

 of objectives, and is slight. Of course, the Ught should 

 not be decreased by lessening the aperture of the condenser. 

 The writer does all the light moderating usually necessary 

 (using a binocular) with the three yellow-green Wratten 

 screens, Xos. 66, 56 and 57A, used singly or two together, 

 with a 108-watt tungsten-ribbon pyrex lamp, and ground 

 glass. Light moderating is an important factor in securing 

 the largest possible cone from the condenser, without flood- 

 ing or glare. It should not be forgotten. 



Improving Existing Outfits. — If the reader cannot get an 

 ideal outfit, he can yet improve his microscope in several 

 ways. First, and best, he can get covers 0.16 to 0.17 

 millimeter thick. This will improve all images. If he 

 has only an uncorrected condenser, as is perhaps probable, 

 since this has been the fashion since Koch's use of it in 1878; 

 then he can use a small electric lamp close below it, with 



