308 W. M. BALE ON THE 



stated as 40 and 80, and with a higher ocular as 156 and 415. 

 Obviously if the substitution of the higher eye-piece doubled the 

 power in the case of the one-inch, as stated, it would do likewise 

 in the case of the quarter, and assuming 156 to be correct with 

 the first eye-piece, the power with the higher would be 312, not 

 415. Actually, the powers of the four combinations, instead of 

 being 40, 80, 156, and 415, as stated, are approximately 43, 75, 

 188 and 327. 



Broadly speaking, there is only one way of measuring the 

 magnifying power of a microscopical combination where the 

 separate powers of the constituents are unknown, namely, by 

 comparing the actual diameter of some object (usually the scale 

 of a stage micrometer) with the magnified image of the same. 

 This is most readily done by projecting the image of the micro- 

 meter-scale by means of some form of camera-lucida on to a paper 

 at a distance of ten inches and marking some of the divisions on 

 the paper, or even by projecting them on to a ruled scale, so that 

 comparison is easy ; e.g. if one-hundredth of an inch when magni- 

 fied equals one inch, the power is 100. 



Thus far the textbooks ; but these directions leave room for a 

 certain amount of error owing to their ignoring the fact that in 

 all camera drawings the magnification is appreciably greater in 

 the marginal portions of the field than near the centre. If we 

 project the micrometer-scale across the field and mark ofi the 

 divisions, we naturall)^ select if possible so many divisions for 

 comparison as will coincide exactly with one or more divisions of 

 our rule, in order to avoid fractions ; and in one measurement 

 we may attain this end by utilising only the central part of the 

 field, while in another we may have to utilise nearly the whole. 

 The result obtained in the first case should be correct ; the other 

 will, with an eye-piece giving a large field, be 4 or 5 per cent, 

 too great. 



The procedure which I adopt to ascertain the magnifying power 

 of a combination is as follows : 



A. — Measure with the camera * the exact diameter (at 10 inches) 

 of the magnified field. 



* A Bcale reflector is recommencled, as it is an appliance so easily 

 improvised if the microscope is not provided with one. The simplest 

 method is to take a piece of wire, wind it two or three times romid the 

 oye-piece, twist the two ends together to keep it firm, and then, keeping 



