38 HOW TO WORK 



power of the different lenses. Although the several object-glasses 

 are termed one inch, one quarter of an inch, one eighth, &c., the 

 magnifying power of each is not definite, and the quarters of some 

 makers magnify with the same eye-piece many times more than those 

 of others. It is well, therefore, that every observer should be able 

 to ascertain for himself the magnifying power of his different glasses. 

 Suppose I wish to know how much a French quarter magnifies. The 

 one-thousandth of an inch micrometer is placed in the field, and the 

 magnified image is thrown by means of the neutral tint-glass reflector 

 upon a scale, divided into inches and tenths of inches placed ten 

 inches below the eye-piece. If the magnified one-thousandth of an 

 inch covers about two-tenths of an inch, the glass magnifies 200 

 diameters ; if it covered one inch, the thousandth of an inch must 

 have been magnified 1,000 times, but in this case it only corresponds 

 to the one-fifth of an inch, and therefore the one-thousandth is 

 magnified 200 times. For lower powers the one-hundredth of an 

 inch scale may be employed. The manner of ascertaining the mag- 

 nifying power is therefore exceedingly simple ; but it is very impor- 

 tant for the observer to know the magnifying power of every lens 

 with each different eye-piece, and he should ascertain this before 

 he commences to make any observations. This simple process 

 will be readily understood if fig. 66, in pi. XIII, be carefully 

 studied. To carry out this plan it is only necessary to be pro- 

 vided with a stage micrometer, divided to icoths and i,oooths of an 

 inch, which can be purchased for $s. o</., and an inch scale divided 

 to tenths. 



G4. To Ascertain the Diameter of an Object. If an object be 

 substituted for the micrometer, and its outline carefully traced upon 

 paper, its dimensions may of course be easily ascertained by com- 

 parison with the micrometer lines. The magnifying power used 

 being the same in both cases. 



In order to apply this plan to microscopical drawings generally, 

 the following seems to be the simplest mode of proceeding, and 

 saves much trouble. Scales are carefully drawn upon gummed 

 paper ; the magnifying power, and the micrometer employed being 

 written upon as represented in pi. XIII, fig. 63. If a number are 

 drawn together, one of the rows can be cut off and appended to the 

 paper upon which the drawing, magnified in the same degree, has 

 been made. The observer may save himself the trouble of drawing 

 these on paper, by having them engraved on wood or stone, and 

 several copies worked off. This is the plan I have followed in 

 all the drawings which illustrate my observations, and the scales 

 have been copied in the wood-cuts and plates. 



