THE FOCUS-APERTURE RATIO. 295 



ject, but I wish to see for myself something of those microscopical 

 things which are at present hidden from me, but my battery must 

 be limited to two objectives and two eyepieces, which should I 

 get? The answer is 1 in. and 1/4 in., with x 5 and x 10 

 eyepieces. Another says the same, but is prepared to have three 

 objectives and three eyepieces ; the answer in this case would be 

 2 in., 1/2 in., 1/8 in.* and X 7^, x 15, X 12§ eyepieces.^ Another the 

 same but with four objectives and four eyepieces ; then the answer 

 would be 2 in., 1/2 in., 1/4 in., 1/8 in., and X 7|, X 15, X 8§, and 

 X 18§ eyepieces. Five objectives form a complete set, the 4-in. 

 and 1/16-in. of key A and the 3-in. of key B may be regarded as 

 luxuries. The above selections apply to general microscopy 

 only. To give an example of special microscopy, suppose a 

 student says,I am going to study bacteria and nothing else, and the 

 limit of my battery is three objectives and three eyepieces, then 

 his best selection would be a 2/3 in., l/3f in., and 1/12* in., with 

 X 7|, X 8§, and x 18§ eyepieces. This is the sort of scale that is 

 to be found in most makers' catalogues, but it is a most unsuitable 

 one for general work. For pond life, with three lenses and three 

 eyepieces, the series would be 1| in., 2/3 in. and 1/3| in., with X 7j, 

 X 15, and x 12§ eyepieces. 



With regard to the foregoing lists there can be "no probable, 

 possible shadow of doubt, no possible doubt whatever ! " as 

 saith the Grand Inquisitor. 



From a very old saw we learn that time is money. Now, much 

 time is spent with a microscope in " finding," and it is obvious 

 that that microscope which shortens this time as much as 

 possible is economically the better instrument. It is also equally 

 obvious that the time spent on " finding " is inversely proportional 

 to the area of the field. We therefore can compare the relative 

 economic values of the English and Continental microscopes, with 

 reference to time saving, by the measurements of fields. With 

 equal powers, the area of the field of the English microscope is 

 about three times that of the Continental, so an object could be 

 found with an English microscope in one -third of the time. 

 Therefore a purchaser may expend three times as much upon 

 an English stand without suffering loss ! An hour spent upon 

 " finding " is by no means an uncommon incident ; if this can be 



^ -j- means apochromat, § compensating, and * oil immersion. 

 JouRN. Q. M. C, Series IT.— No. 88. 21 



