86 



THE MICROSCOPE 



In practice the matter is more complicated than it appears, 

 because the thickness of the lenses not only alters their foci 

 but also slightly alters the ratio of their partial dispersions, and 

 this has to be allowed for to get the corrections accurate. 



In achromatic object glasses it is usual to correct for about 

 C to F ; this means that when correct visually there is a very 

 slight error for D ; but these can be neglected for most work, but 

 for photographic purposes the error in G might be appreciable 

 tinless a coloured screen is used. 



In apochromatic object glasses the colour is corrected for at 

 least three parts of the spectrum, and also the spherical aberration 

 is much more fuUy correctedjfor all colours ; this means the lower 

 power lenses have decidedly larger apertures than the correspond- 

 ing achromatic lenses and^ that with all apochromatic object 

 glasses higher eyepieces can be used. Also, when it is important 

 to distinguish between objects with slight differences of colour, 

 these lenses are much to be preferred. 



li-inoh 

 object glass. 



S/3-indi 

 object glass. 



l/S-inoh 

 object glass. 



Chaeacteeistics of Object Glasses of Different Powers 



The IJ-inch (40-mm. or 32-mm.) object glass gives a maximum 

 field of view "2 inch (5 mm.) and has a working distance of 

 •88 inch (22 mm.) in the achromatic, or 2-05 (50 mm.) in the 

 apochromatic series. It is, therefore, specially useful for obtain- 

 ing a general view of large entomological and botanical specimens. 

 It is the only object glass with which opaque vertical reflectors of 

 the type of the Sorby flat silvered mirror, or the parallel flat 

 glass mirror, can be used between the front of the object glass 

 and the specimen, and is, therefore, useful for low-power metallur- 

 gical specimens. Its aperture (-16 or -17 N.A.) gives a theoretical 

 resolution of 15,000 to 18,000 lines to the inch, according to the 

 colour of the light employed. 



The 2/3-inch (16-mm. or 14-mm.) object glass gives a maximum 

 field of view of -08 inch (2 mm.). The achromatic 16 mm. 

 has a working distance of -25 inch (6J mm.). The apochromatic 

 16 mm. and 14 mm. have working distances of -ll inch (3 mm.) 

 and -07 inch (1| mm.) respectively. This is the power that is 

 the most useful all-round low-power lens for every purpose. It 

 forms a useful finder for searching specimens to be examined 

 later with a high power. It can be used with the parabolic 

 reflector for opaque objects, and is probably used in larger numbers 

 than any other lens. The aperture of the achromatic (-28 N.A.) 

 gives a theoretical resolution of 25,000 to 30,000 lines, and of the 

 apochromatic -35 N.A., 30,000 to 36,000 lines to the inch. 



The 1/3-inch (8-mm.) object glass gives a maximum field 

 ofview of -045 inch (ij mm.). It has a working distance of 

 about -OS inch (1| mm.). It is a medium-power lens of the greatest 

 use for many purposes, and is notjsufficiently appreciated. For 



