INITIAL MAGNIFYING POWERS OF OBJECTIVES. 209 



The stage micrometer is placed on the stage, and the constant 

 of the screw-micrometer determined. The focus of the micro- 

 scope is not to be disturbed, but, by means of substage focusing, 

 the lens to be measured is racked up until the image of the card 

 is sharply focused. Then one of the sides of the card is spanned 

 by the webs of the eye-piece micrometer, and its size measured 

 and the magnifying (or rather diminishing) power found : then 



._ 100 

 ~~ m + '2' 



Of course, the idea of the 5 inches is that the reading is 



doubled, and then 10 -t- x (say), gives the magnification, m, which 



can be found from reciprocal tables, as well as the value of 



^ ' 1,1 + T 



It is not difficult, but a little more trouble, to make the calcu- 

 lations without tables. 



For the benefit of photomicrographic members, the following 

 is quoted from a note by Mr. Nelson. " This method will 

 measure the foci of large photographic lenses. In that case 



,_ 100 _ 100 



m + 2 (m + If 



" This second term is only necessary when f is large compared 

 with one hundred inches ; for microscopic lenses it is not wanted. 

 The whole can be determined from reciprocal tables without 

 putting pencil to paper." The tables referred to are those of 

 Barlow, published by Messrs. E. & F. N. Spon. 



The screw-micrometer eye-piece is, perhaps, a drawback. Mr. 

 Nelson says, "An ordinary screw-micrometer with a negative 

 eye-piece is no good for lens measurements ; the eye-piece must be 

 of the Ramsden type, and it is very doubtful if any ordinary 

 ruled glass micrometer eye-piece would be sufficiently accurate. 

 A screw-micrometer is necessary for both the methods described 

 in the paper." 



