64 Modern Microscopy 



3, 4, etc. This is not very intelligible, and it would be far 

 better either to express their equivalent focus, as in the case 

 of objectives, or to have the magnifying power in diameters 

 marked on the cap. We will take it that the ' A ' eyepiece 

 yields a magnification of 5 diameters. When this, there- 

 fore, is used in conjunction with the 1-inch objective on a 

 10-inch tube-length, which according to the rules previously 

 given would produce a magnification of 10 diameters, the 

 resultant combined power is 50 — that is, the powers of the 

 objective and eyepiece multiplied together. The method of 

 estimating the power with short tube-lengths is referred to 

 on page 69. 



OBJECTIVES. 



For our purpose we shall divide the subject of objectives 

 into two classes — (1) the apochromatic, and (2) the achro- 

 matic. The immersion objective which may belong to 

 either of these two classes is referred to separately. As in 

 our remarks on objectives we shall constantly use the two 

 terms, we will describe their reference. 



Apochromatic Objectives. — The introduction of these 

 objectives by the firm of Carl Zeiss, of Jena, Germany, 

 placed the science of microscopic optics on a far higher 

 level than had hitherto been attained, and as a result 

 many of the traditional modes of working have been 

 altered. Greater precision has been necessitated in the 

 microscope - stand, and the provision of sub -stage con- 

 densers of corresponding optical quality to the objectives 

 has been essential. Professor Abbe and Dr. Schott were 

 granted a subsidy by the Prussian Government with a view 

 to the promotion of optical research, and aided by this they 

 were able to produce several varieties of new optical glass. 

 The employment of these new glasses in conjunction with 

 fluorite, based upon the careful and elaborate calculations of 

 Professor Abbe, resulted in the production of apochromatic 

 objectives. In these lenses, aberrations which were inherent 

 in the older systems were eliminated or minimized — that 



