OBJECT GLASSES AND EYEPIECES 91 



The difficTilties of using an oil-immersion lens, however, chiefly 

 apply to cases where it is necessary to search with a low power, 

 change to a high power, and then rapidly search again, as in the 

 latter process the oil must be wiped off the cover glass with a 

 piece of filter paper dipped in benzol or xylol before the low power 

 is used. The 1/6-inch dry apochromatic with a cover glass 

 adjustment is a very useful lens for such work, because it has a 

 very large aperture and resolving power, and if carefully adjusted 

 for the cover glass thickness, owing to its very perfect corrections, ■ 

 can be employed with very high eyepieces to do much of the 

 work that would otherwise be done with a 1/12-ineh. 



For botanical work the 2/3-inch (16-mm.) and 1/6-inch (4-mm.) Botany. 

 are generally used by the student. A great deal of the work 

 could be better done with IJ-inch (32-min.) and 1/3-inch (8-mm.) 

 with a higher eyepiece. A 1/12-inch oU-immersion may be added 

 for cell structure, such as Karyokinesis. 



For metallurgical work the best three lenses are the 1 J-inch MetaUurgy. 

 (32-mm.), 1/3-inch (8-mm.) and 1/12-inch (2-mm.) oU-immersion. 

 For photography the apochromatic series have a marked advantage, 

 as colour screens need not be used ; compensating eyepieces 

 should always be selected for photomicrography as the Huyghenian 

 eyepieces are not satisfactory for this purpose. For chemical 

 and industrial purposes it is difficult to make any recommendation. 

 The objects examined are so varied and the conditions so diSerent 

 that nothing but a complete series will meet every requirement. 

 It is best to study the capabilities of each object glass as given 

 on page 86, and select according to circumstances. 



For petrology the best two lenses are the 2/3-inch {16-mm.) Petrology, 

 and the 1/6-inch (4-nim.) ; the high-power dry lens being essential 

 for observing interference figures, as a large aperture is necessary 

 for this work. A low-power 1^-inch (32-mm.) is very useful. 

 The apochromatic series must be used with caution for this 

 purpose, the fluorspar of which they are made may render them 

 unsuitable in some cases. 



For general recreation the whole series wiU appeal to the oenerai 

 microscopist who wishes to dip into a large number of subjects. '■^°"**'™- 

 If only two object glasses are required he should begin with a 

 l|-inch (32-mm.) and a 1/3-inch (8-mm.) and three eyepieces. 



