OBJECT GLASSES AND EYEPIECES 87 



bacteriological and pathological investigation it can be used to 

 do a great deal of the work for which two object glasses are 

 usually employed. With a low-power eyepiece it has a field of 

 view large enough for searching, and with a high- power eyepiece 

 it can be used for blood counts and recognising micro-organisms 

 such as trypanosomes, malaria parasites, and bacteria, and for 

 pond life it is perhaps the most useful all-round power. For 

 metallurgy it is an excellent lens for photography, and with this 

 lens and an oil-immersion lens a microscopist can frequently do 

 all his work in certain branches of research. This lens is par- 

 ticularly useful for dark-ground illumination with a substage 

 condenser and patch-stops. The achromatic with an aperture 

 of 5 N.A. gives a theoretical resolution of 48,000 to 52,000 lines, 

 the apochromatic of '65 N.A. about 62,000 to 67,000 lines to 

 the inch. 



The 1/6-inch (4-mm.) object glass gives a maximum field of i/e-inch 

 view of -02 inch (1/2 mm.) and has a working distance of -024 inch ° ^^^ ^ ^^' 

 (1/2 mm.). It is the universal high-power, and when only two lenses 

 are supplied, for cell- structure, histology, and all general high- 

 power purposes, it is more popular than any other lens, and being 

 made in large quantities is moderate in price. The achromatic 

 has an aperture of -85 N.A., the apochromatic -95 N.A., and 

 theoretical resolving powers of about 81,000 to 88,000 lines and 

 90,000 to 100,000 lines to the inch respectively. The very 

 large aperture of the apochromatic lens of this focus and its perfect 

 corrections renders it specially valuable for use with high eyepieces 

 when an oil-immersion cannot be used. In this case the 1/6-inch 

 with a correction collar should be selected, and the microscopist 

 should become familiar with the correct adjustment of this 

 collar (see page 80). 



The 1/8-inch (3-mm.) oil-immersion object glass has a maxi- i/8-inch 

 mum field of view of -015 inch (-STS-mm.) and a working distance of °^J^^*s ass. 

 •016 inch (•4-mm.). Being an oil-immersion lens, it is not affected 

 by the thickness of the cover glass used, and is thus always 

 working at its best. It is introduced not because a power between 

 a 1/6-inch (4-mm.) and a 1/12-inch (2-mm.) is often required, but 

 because a special lens with a maximum aperture that can be 

 used with dark-ground illumination is urgently required for this 

 work. As explained in the description of the use of the dark- 

 ground illuminator, a large aperture oil-immersion lens such as a 

 1/12-inch (2-mm.) 1^3 N.A. must be stopped down to the aperture 

 of a 1/6-inch (4-mm.) dry lens to enable it to be used with dark- 

 ground illumination, and resolving power is thus lost. The 

 1/8-inch (3-mm.) object glass can also be used to do most of the 

 work with one object glass that is generally done with the 1/6-inch 

 (4-mm.) and the 1/12-inch (2-mm.). It has an aperture of -95 

 N.A. and a theoretical resolving power of 90,000 to 100,000 lines 

 to the inch. 



