THE OBJECTIVE 111) 



High-power, water-immersion objectives are not offered 

 by all makers. The 2-millimeter achromatic and the 

 2.5-millimeter apochromatic water-immersion objectives 

 of Zeiss have correction collars, and it needs some practice 

 before expertness in their use is attained. Leitz has a 

 2-millimeter, water-immersion objective without a correc- 

 tion collar, and so it can be corrected only by unscrewing 

 more or less the front two lenses, or using only covers 0.17 

 milhmeter thick. (Spencer also makes a high-power, 

 water-immersion of 1.0 aperture.) The use of water- 

 immersion objectives is advocated by the writer on objects 

 in a layer of water more than 10 microns deep, as already 

 stated more than once. 



Apertures and Magnifications. — The excess aperture 

 of an oil-immersion objective over 1.0 cannot be illuminated 

 by rays of light in air. Consequently, on holding such 

 an objective up to the light of a window, and looking 

 through it from the back, at the front lens, with a 3- or 

 4-times magnifier, a dark ring is seen round the illuminated 

 part. This ring is due to the excess of aperture over 1.0. 

 This dark ring of course becomes relatively wider as we 

 pass from a 1.05 objective to the 1.25, 1.3, and 1.4 oil- 

 immersion objectives. 



The due ratio of aperture to focal length was discussed 

 by Abbe (14). Since the aperture of an objective deter- 

 mines the limit of useful magnification, we should calculate 

 for each objective the highest magnification which can 

 advantageously be used. In the modern way of calculating 

 magnifications, the objective is credited with the magnifi- 

 cation given to the image which would be formed by it 

 if the Huyghenian eyepiece were removed (or the image 

 which is formed in a magnifier eyepiece, like K 15 of Zeiss), 

 the rest of the total magnification being reckoned as due 

 to the eyepiece. Several optical firms now mark the objec- 

 tives with these initial magnifications, by which numbers 

 the objectives can be conveniently designated instead of 

 by their focal lengths. The latter had gradually become 

 more or less erroneous; many objectives having true focal 



