64 THE USE OF THE MICROSCOPE 



lens might also be supplied to make the definition perfect. 

 The writer's experience has seemed to show that the 

 trouble in continuous counting of corpuscles was due 

 to optical errors caused by the thick cover. 



Summary. — Often a routine worker does not need the 

 full capacity of his objectives, and regularly uses a low 

 eyepiece (5 times or so). But high eyepieces (of 10 or 

 15 times) can now be obtained with large upper lenses 

 and comfortable eyepoints. Such eyepieces are compen- 

 sating (or orthoscopic). Hence, in the case just mentioned, 

 objectives of lower aperture can be used with as good 

 results as are attained by cutting down high apertures, 

 and with more comfort in their use. Thus the 40 dry 

 objective may often be advantageously replaced by the 

 20, and the 90 oil-immersion objective by the 50 oil-immer- 

 sion of lower aperture. A dry achromatic condenser is 

 preferable, from every point of view, to the uncorrected 

 ''Abbe" condenser. With dayhght, a large card dia- 

 phragm on the window helps to prevent glare. A con- 

 venient electric lamp is a C-Mazda of 100 watts, matt 

 inside. Diaphragms should be placed close in front of 

 this lamp. If the uncorrected condenser is retained, it 

 should be used in connection with a bull's eye, or ground 

 glass, as explained in the following chapter. It pays to 

 pick out (or purchase) covers nearly 0.17 millimeter thick, 

 for the best work. 



Practical Points 



1. Eyepieces of 10 and upwards are now made with high 

 enough eyepoints (orthoscopic or compensating). Therefore 

 higher eyepieces than 5 times can be regularly used with low 

 and medium objectives; and, with a good condenser, with high 

 objectives. 



2. A dry achromatic condenser, of nearly 1.0 maximum 

 aperture, such as is now made by several manufacturers, is, as 

 is well known, superior enough to the uncorrected condenser, for 

 careful microscopy, to be worth the $10 or so of extra cost. 



3. The slight extra cost of an achromatic condenser is bal- 

 anced by the extra performance got out of the objectives, and 



