41 
First Sur- 
Aperture. T=1, Uncovered. | Covered. | face ae 
20° 1:74 1:62 1:53 1:52 
40° 6°91 6°47 6:07 6:06 
60° 15°35 14:37 13°47 13°46 
80° 26°81 25°07 23°45 23°46 
100° 40:93 38°12 35°52 35°60 
120° 57°30 52-90 48°86 49°21 
130° 66°16 60°38 55°33 56:14 
140° 75°40 67:93 61°89 63.05 
145° 80°13 71.25 64:70 66°39 
150° 84:93 75°23 67-26 69°69 
155° 89°79 78:09 69-70 72°83 
160° 94-69 81:68 71:54 75°89 
165° 99°63 84:77 73:11 78°73 
170° 104:60 86:56 73°97 81:52 
175° 10959 88°43 74:33 84:17 
180° | 114:59 88-60 74:52 86°84 
‘‘ These numbers (which, from neglecting the remaining 
reflexions, must give rather too much weight to the larger 
apertures) show clearly that, especially for covered objects, 
nothing is gained above 150° at all commensurate to the dif- 
ficulty of constructing such objectives. But in addition to 
this, I wish to call attention to the fact, that the whole of 
these great apertures is not in every case thoroughly effective. 
“The mode of measuring them which is commonly used 
is that given by Mr. Lister, in which the microscope is at- 
tached to the alidad ofa circular instrument, with its objective 
over the centre, and directed towards a luminary at some dis- 
tance. Looking into it, the field is filled with light, which, 
on turning the alidad, is seen to have a circular boundary: if 
this be brought from each side to the middle of the field, the 
intercepted arc is the aperture.* For objectives of considerable 
* When the aperture is very large, there is a sluggishness in the appa- 
rent movement of the boundary, which makes me have some doubt of the 
accuracy of the process ; it seems almost stationary in No. 6. 
