274 



A?)iencan Quarterly Microscopical Journal. 



sine of half the aperture, observed when the shutter rests on 

 that point, by the index of refraction of crown-glass. By this 

 method a calculation, or the use of a specially constructed table, 

 is ahvays necessary to translate the reading into degrees, not 

 merely to obtain the aperture in air or water, but also to obtain it 

 in crown-glass, the medium through which it is actually measured. 

 Moreover, the divisions of his arbitrary scale are so far apart that, 

 for example, the distance from i.io to 1.15, the next higher divi- 

 sion, corresponds to the difference between the half apertures 

 46° 3' and 48° 56', making a difference of 5° 46' between the 

 two apertures indicated, supposing the semicylinder to be made of 

 crown-glass of an index of refraction of 1.525 ; and in the case of 

 the higher readings of his scale the differences will be still 

 greater. There is no means of measuring the intermediate points 

 on his apparatus, they can only be estimated ; that is, guessed at. 

 As a consequence, we have the strange anomaly offered in his 

 paper {pp. cit., p. 21) of a balsam angle computed at ninety-four 

 degrees and thirty minutes, from the reading of a scale the units 

 of which are upwards of five degrees each. Moreover, even should 



Fig. II. 



