48 Modern Microscopy. 



power eyepieces, it is advisable for ordinary achromatic 

 lenses that no stronger power should be used than an 

 eyepiece giving an initial power of 15 diameters. The best 

 eyepiece for general purposes is the B. This gives a nice 

 size field, and is by far the most comfortable of the whole 

 series. Next, and in addition to this, we should recommend 

 either the C or the D. 



Compensating Eyepieces. 



Under the head of ' Objectives,' it will be seen that we 

 refer to the new apochromatic objectives introduced by Zeiss, 

 and made by several other firms. In order to obtain the 

 best results, it is necessary to use with them compensating 

 eyepieces. These are over-corrected, and the apochromatic 

 objectives under-corrected, while the Huyghenian eyepieces 

 are under - corrected, and the ordinary objectives over- 

 corrected. It is obvious, therefore, that Huyghenian eye- 

 pieces are unsuitable for the apochromatic objectives ; also 

 the compensating eyepieces will not work well with objec- 

 tives of low power, but they give very good results with 

 objectives of large aperture of the achromatic series ; still 

 it would not be worth while to purchase them for use 

 with these. These eyepieces being achromatized, and con- 

 sequently having more lenses in them, are more expensive 

 than the Huyghenian form. 



Kelner Eyepieces. 



This is another achromatic form of eyepiece, giving 

 an exceedingly large field, and considerably used for the 

 examination of animalculse, pond life, etc. A certain 

 amount of definition is, however, sacrificed with them, and 

 although we occasionally use them, before deciding on pur- 

 chasing any, we should recommend the microscopist to 

 judge for himself as to the desirability or otherwise of his 

 having them ; they are not by any means necessary 

 adjuncts. We may mention that they give very good 

 results photographically. 



