DETERMINATION OF THE MAGNIFYING POWER. 225 



ascertained by the application of a pair of compasses ; and, if 

 necessary, the squares, as shown in figs. 146 and 147, can be 

 further subdivided into four, so that the diameter of an object 

 can be measured even to the fourth part of the quantity given 

 by the lines on the micrometer. The whole field of view need 

 not be covered with lines, .as shown by the figures; but a 

 single square, or even the exact distance of any twO' or more 

 of the lines of the micrometer, together with a pair of com- 

 passes, will be all that is required. It must, however, be 

 borne in mind, that when the size of an object is ascertained 

 by the above method, the distance between the camera 

 and the table must be always the same ; if the end of the 

 compound body carrying the camera were ever so slightly 

 raised or depressed, there would be either an increase or 

 diminution of the value of the squares of the micrometer. 



CHAPTER IV. 



ON THE METHODS OP OBTAINING THE MAGNIFYING POWER 

 OF SINGLE AND COMPOUND MICROSCOPES. 



The method of estimating the magnifying power, either of 

 simple or compound lenses, is to compare an object of known 

 size, such as a micrometer, seen through them with the nearest 

 distance that another object, also of known size, can be dis- 

 tinctly seen at : this latter distance is termed the standard of 

 distinct vision, and with it all the magnifying powers are com- 

 pared. All opticians of the present day adopt ten inches as 

 a standard ; Sir David Brewster adopts five inches. In the 

 old optical works, eight inches were generally employed as the 

 standard; but the number ten, being a decimal, will be 

 found most suitable for all purposes. With this decimal 

 standard, the magnifying power of lenses, of any focal length, 

 can be readily determined. Thus, for instance, if the lens 

 uuder examination be of one inch focus, we have merely to 

 15 



