Objectives. 39 



the object, but very approximately the actual magnifying 

 power of the objective, and is based on the power afforded 

 by a simple lens of that focus. For instance, a simple lens 

 of 1-inch focus has a power of 10 diameters, and an objec- 

 tive called a 1-inch, and composed of several lenses in order 

 to produce achromatism, should have a similar power; 

 the J-inch 20 diameters, the J-inch 40, etc. The image 

 formed by the objective is again magnified by the eyepiece. 

 Unfortunately, the latter is rarely marked with its power, 

 the general rule bein^ to call them by the letters A, B, C, D, 

 etc., or 1, 2, 3, 4, etc. This is not very intelligent, and it 

 would be far better to either express their focal power, as in 

 the case of objectives, or to have it marked on the cap in 

 diameters. We will take it that the A eyepiece yields a 

 magnification of 5 diameters. When this, therefore, is used 

 in conjunction with the 1-inch objective, which, as we have 

 said, has a power of 10 diameters, the resultant power is 50 ; 

 that is, the powers of the objective and eyepiece multiplied 

 together. 



OBJECTIVES. 



The enormous strides that have been made in the perfec- 

 tion of objectives within recent years have revolutionized 

 microscopy, and the introduction of the apochromatic 

 objectives has put the science of microscopic optics on a 

 higher level than had hitherto been attained. Undoubtedly, 

 by critical comparison, these lenses in conjunction with 

 compensating eyepieces, referred to on page 48, are superior 

 to the ordinary achromatic objectives, as in them residuary 

 aberrations are eliminated in a far higher degree than in 

 the best of previous objectives. Credit for this is due to the 

 firm of Carl Zeiss. Subsidised by the German Govern- 

 ment, new optical glass was made, placing in the hands of 

 the optician a greater field from which to select the 

 materials of which to construct his lenses, and by the most 

 careful mathematical calculations the matter was accom- 



