ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



397 



aim of the improvement being to read off theN.A. with greater ease and 

 accuracy. Distinctness and clearness of reading have been effected 

 by increasing the number of marked values of N.A. from 9 to 22, 

 without the confusion that overcrowding of the lines would entail. To 

 accomplish this, short arcs of circles are used instead of whole circles. 

 A valuable property of these is the clear visibility of the ends or edges 

 of the arcs ; they are seen more distinctly than complete circles would 

 be. The contrast between the white ground and the short black lines 

 favours this. The exterior edges of the arcs denote the N.A., and thus 

 give most convenient, accurate, and definite positions for reading. 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 09 N.A. 



The apparatus consists of an apertometer diagram (fig. 35), printed 



apertometer olagram 



Fig. 35. 



on a small card about the same size as 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 diam. Nelson lays 

 some stress on the hole in the diaphragm being not more than 1*25 mm. 

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

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

 is, relatively, more important with the small apertures, because, for 

 example, an error of 0*01 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 compared with the N.A. 0-11 and 

 0*13 of a 3-in. objective, the actual difference between the two pairs of 

 values is * 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." 

 Nelson also makes 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 



