Eyepieces. 



47 



angulatum, and the markings on scales of Podura are 

 suitable. 



EYEPIECES. 



The eyepiece commonly used with the 

 microscope is what is termed the Huy- 

 ghenian form, which consists of two 

 plano-convex lenses placed at a distance 

 apart equal to half the sum of their foci, 

 with a stop in the principal focus of the 

 eye-lens. This will be found to meet 

 all ordinary requirements of micro- 

 scopical work. They vary in power, and 

 these powers are usually designated by 

 the letters A, B, C, D, etc., A being the 

 weakest power. On the Continent they 

 are generally designated 1, 2, 3, 4, etc., 

 while some firms express their power in 

 focal units for instance, an eyepiece having a power of 

 10 would be 1 inch. It is often remarked that the Con- 

 tinental objectives stand a stronger power of eyepiece 

 than the English, and on this account a superiority has 

 been claimed for them ; but it should be borne in mind 

 that no two series of microscope-makers' eyepieces are of 

 the same power, and where English manufacturers give in 

 many instances as deep a power of eyepiece as 20 or 25, the 

 Continental manufacturers make their eyepieces much 

 nearer one another in power, and their deepest eyepiece 

 has usually an initial power of 10 or 12.' A comparison, 

 therefore, between the merits of an English object-glass 

 tested with, say, a D English eyepiece and a Continental 

 object-glass of the same power tested with a No. 4 eye- 

 piece of Continental make would not be fair, as the former, 

 having a deeper-power eyepiece on it, would be liable not 

 to give such perfect results as the latter ; and here we 

 may mention that, although people very often buy deep- 



