56 THE USE OF THE MICROSCOPE 



lar; and it has, in addition, the above-mentioned advantages 

 of the monocular. A miniature electric bulb may be put 

 on a white background below the ground glass covering 

 the unused eye. If green light is used in the microscope, 

 a green screen may be employed here, also. The eye in 

 use should be well shaded from front light by a special 

 opaque screen. 



The Drawtube. — The drawtube is probably usually 

 unnecessary in microscopes for routine work, where it 

 seems often to be a source of error. It may be often 

 better to change the cover-glass than to change the length 

 of the drawtube. For accurate microscopy, as Nelson 

 pointed out (46), there should be an appropriate diaphragm 

 in the drawtube; as otherwise some fogging with light 

 reflected from the sides of the tube takes place. If there 

 is a diaphragm on the source of hght, however, restricting 

 this to the size of the source-field, much reflection does not 

 occur. The tube of the monocular, for accurate work with 

 high dry objectives, was sometimes fitted with a rackwork 

 (Nelson, Coles) for easy adjustment of the tube length. 

 This method, however, appears antiquated, and it seems 

 better to use only cover-glasses 0.17 millimeter thick for 

 important objects. The adjustment for cover thickness 

 is especially important for the apochromatic objective 20, 

 the achromatic 40 without a correction collar, and the 60 

 apochromatic oil-immersion objective. (In view of certain 

 trials made by Ainslie (46), it might be worth while to 

 test some oil-immersion objectives for the best tube length.) 

 Of course, with objects in water, the tube length should 

 be made somewhat longer; and with objects in "hyrax" 

 (or other high-refractive medium) somewhat shorter, 

 even with oil-immersion objectives. Oil-immersion objec- 

 tives must be corrected for cover thickness, for the very 

 best results, unless covers of 0.17 millimeter in thickness 

 are exclusively used. In the writer's experience, the wide 

 microscope tube has no advantage over the ordinary tube 

 for high-power photography, if the source of hght is made 

 nearly equal to the source-field, as defined above. 



