DETERMINATION OF THE ANGLE OF APERTURE. 175 



This method has the great advantage, that not only is the 

 aperture of the objective determined (that is, as far as it admits 

 light), but also the extent of the really effective part, i.e., the part 

 which produces images free from aberration. It is, indeed, fre- 

 quently observed that the images are indistinct and distorted 

 before they disappear, while they seem otherwise sharply outlined 

 and in their true shape. These indistinct and distorted images 

 are, of course, produced by the marginal rays, for which the 

 objective is no longer sufficiently corrected, and which are therefore 

 better cut off. 



The really effective part of the angle of aperture is consequently 

 determined by the limits within which the flame appears sharp 

 and clear. 



(3.) Robinsons Method. 1 A pencil of parallel rays is projected in 

 the direction of the axis through the eye-piece, and, after the 

 crossing of the rays in the focal point of the Microscope, is 

 received on a screen. 2 From the diameter d of the circle of light 

 which is here formed, and from the distance I of the screen from 

 the focal point of the Microscope (or the anterior end of it, if 



method could not yield quite similar results to those of Lister, because by the 

 addition of a magnifying-glass the principal focal point was displaced, is at 

 any rate inaccurately explained. It is, on the contrary, quite immaterial 

 whether the images a'" and b'" are viewed with the naked eye, with a magnifying- 

 glass, or with a second Microscope. Their disappearance in no way depends 

 upon the additional system of lenses. Nevertheless, a slight difference does 

 exist, though the cause is not as Harting supposes. With Lister's method the 

 pencils of rays are refracted in the margin of the Microscope so that they cut 

 the axis in p' ; it is only on this condition that the centre of the field of view is 

 seen illuminated. In Wenham's method, on the other hand, it is sufficient that 

 the pencils reach the field-lens after refraction in the objective a condition 

 which is still fulfilled if their path is at some distance from the axis. The 

 angle of aperture is somewhat larger than it would be if the incident cone of 

 light emanated from the focal point of the objective. 



1 Vide " Proceedings of the Royal Irish Academy," v. (1858), pp. 3847, 

 " On a new method of measuring the angular aperture of the objectives of Micro- 

 scope," by the Rev. T. R. Robinson, D.D., President of the Royal Irish Acad., 

 communicated Jan. 23, 1854 ; partly reprinted in " Quart. Journ. Micr. Sc." ii. 

 (1854), p. 295. [ED.] 



2 Dr. Robinson stated that for measuring very large apertures he " was 

 obliged to modify the method by receiving the light on a screen made to travel 

 in a cylindric surface concentric with the focal point," which he considered a 

 decided improvement. Vide " Quart. Journ. Micr. Sc." iii. (1855), p. 164. 



[ED.] 



