14 The Microscope 



purposes. They are usually made with relatively low numerical apertures 

 and long working distances. 



Apochromatic objectives are made on an entirely different principle 

 and must be used with special eyepieces. The colors are intentionally 

 widely separated— or "overcorrected"— and then brought back to a nearly 

 common focus by a "compensating eyepiece." An apochromatic objective 

 used with an ordinary eyepiece gives a much worse image than the much 

 cheaper achromatic. Apochromatic objectives are available only in higher 

 magnifications, usually from X20 up, with very high numerical apertures 

 and correspondingly short working distances. Their use is only justified 

 in photography, particularly color photography, and in the most critical 

 research. 



OCULARS 



There are, as indicated in the last paragraph, two main series of 

 oculars: the regular, or Huygenian, ocular for use with achromatic ob- 

 jectives, and the compensating ocular for use with apochromatic objec- 

 tives. Each of these series is available in a number of "powers" or magnifi- 

 cations, of which the X 10 is standard with most manufacturers. Oculars 

 of lower power than this are of little practical value, except on the rare 

 occasion when an increased field of view is required. Oculars of higher 

 power, particularly the X12.5 and Xl5, are often useful to reduce eye- 

 strain when making continued examinations of small objects. It must be 

 remembered that these oculars add nothing to the resolution of the sys- 

 tem but only increase the apparent size of objects already resolved by 

 the objective. The salivary chromosomes shown in Fig. 144 are a good ex- 

 ample. A good X20 (8-mm) objective clearly resolves even the finest 

 bands on the chromosomes, but it is irksome to count and compare the 

 bands at a magnification of X200. The substitution of a Xl5 eyepiece to 

 raise the total magnification to X300 adds nothing to the knowledge, but 

 a great deal to the comfort, of the observer. 



Too much attention should not be paid to "flatness of field" for visual 

 observation. The eye observes only a small portion of the field of view 

 at one time and a touch on the fine adjustment will bring the periphery 

 into focus. Flatness of field is, however, vital in photomicrography, and 

 the special eyepieces designed for this use are discussed in the next 

 chapter. 



SUBSTAGE CONDENSERS 



The substage condenser is just as vital to resolution as is the objective. 

 Broadly speaking, two types are in use today— the Abbe and the Achro- 

 mat. This is rather confusing since the first type is commonly employed 

 with achromatic objectives and the second with both apochromatic and 



