Eyepieces 77 



Spencer and Smith Optical Company having high reputa- 

 tions for their lenses. 



Of course, different requirements would necessitate the 

 selection of special objectives, but a practical microscopist 

 or any microscope manufacturer would be able to advise on 

 the matter. 



EYEPIECES. 



The eyepiece commonly used with the microscope is 

 what is termed the Huyghenian form, which generally 

 consists of two plano-convex lenses placed at a distance 

 apart about 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 microscopical work with 

 achromatic objectives. Eyepieces vary in power, and these 

 powers are usually designated by the letters A, B, C, T>, 

 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. 

 This last method, or that adopted by 

 Zeiss for the compensating eyepieces, 

 and several progressive English houses 

 for their ordinary eyepieces, where the 

 actual magnifying power is engraved on 

 the cap of the eyepiece, is the only 

 rational one. The letters A, B, C, etc., or 

 Nos. 1, 2, 3, etc., convey no real idea of 

 the magnifying powers of the eyepieces. Fig. 20.— Hut- 

 because each maker has his own formula "^henian Eyepiece. 

 for each eyepiece, and there is no correspondence in the 

 powers of one eyepiece marked ' D ' by one maker, and that 

 supplied by another. It is often remarked that the Conti- 

 nental objectives stand a stronger power of eyepiece than the 

 English, and on this account a superiority has been claimed 



