282 E. M. NELSON, AN IMPROVED FORM OF CHESHIRE'S APERTOMETER. 



The first or lowest marked value is 0'05 N.A., and the values 

 increase by increments of 0'05 up to 0'5 N.A. From 0*5, the 

 values increase by 0*033 up to 0*9 N.A. 



The apparatus consists of an Apertometer diagram (fig. 4) 

 printed on a small card about the same size as Mr. Cheshire's 

 form, another card of explanations and instructions, a cubic inch 

 of wood and a metal diaphragm with a hole not more than 

 1*25 mm. in diameter. Mr. Nelson lays some stress on the hole 

 in the diaphragm being not more than 1'25 mm. in diameter. 

 He says: "If the hole is larger than that, some objectives, 

 especially low powers, will read a great deal too high. And 

 accuracy is, relatively, more important with the small apertures, 

 because for example an error of O'Ol or 0*02 will make a far 

 greater percentage of difference than it would with, say, the 

 N.A. of an oil-immersion objective. If 1*25 and 1*27 be com- 

 pared with the N.A. 0*11 and 0*13 of a 3-inch objective, the 

 actual difference between the two pairs of values is 0*02 in each 

 case, but the percentage difference with the higher N.A. is only 

 1*6 as compared with 18 in the case of the low values." 



In this connection, Mr. Nelson has made another important 

 remark, namely, " The working aperture is larger than the 

 correctly measured true aperture, so that low powers resolve more 

 than they are entitled to theoretically. This is probabhy due to 

 the practically enlarged aperture caused by the rolling motion 

 of the eye from side to side." 



It will also be noticed that the diaphragm to be used with the 

 apertometer is made convex on one side, and if the convex side 

 is put into the larger aperture of an eye-piece or other 

 diaphragm, it rests steadily in position. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XII. , Ko. 74, April 1914. 



