448 Major L. Darwin. On the Method of 



is a sheet of cardboard with a small hole in the centre at H, and this 

 screen, hole and all, is covered with thin white paper on the side 

 away from the lens ; the distance between H and N is always made 

 equal to the principal focal length of the lens ; the bar D is made 

 to cast a shadow from the movable lamp M on the paper just over 

 the hole in the cardboard ; thus, in this shadow, the paper is illu- 

 minated entirely by transmitted light from the lens, whilst the 

 paper round it is illuminated entirely by the light of the movable 

 lamp. 



An obsei-vation is made in the following manner : The lens is 

 first placed in such a position that its axis passes through the hole H ; 

 the lamp M is then moved backwards or forwards until the trans- 

 mitted illumination of the paper at H is made to match as nearly as 

 possible the reflected illumination of the paper round it; the dis- 

 tance between S and M is then noted. The lens is now placed in the 

 position shown in fig. 23, where AB represents the length of the 

 diagonal of the plate for which the lens is being examined, and where 

 the angle is half the angle of field under examination. The 

 balance of light is readjusted by a movement of the lamp, and the 

 distance MS is read off a second time. By finding the inverse ratio 

 of the square of these two readings, we thus obtain the ratio between 

 the illuminations at P and H, the lens being in the position shown in 

 the sketch, and the object being supposed to be equally illuminated 

 in both cases. But what is wanted is the ratio between the illumina- 

 tions on the plate at P and A ; this is found with perfect accuracy by 

 multiplying the ratio of the illumination at P and H, as above ob- 

 tained from the observations, by cos 3 0, and this result is that which 

 is entered in the Certificate of Examination. The relative illumina- 

 tion of the centre and of any part of the field can, of course, be ob- 

 tained in this manner, in the above instance the corner of the plate 

 being the point chosen. 



This test may with advantage be made with the largest stop sup- 

 plied, and also with the stop which has been shown, under test No. 

 13, to give good definition over the whole plate. 



It cannot, however, be denied that there are objections to this 

 method of examination. The fact that the illumination of the plate 

 is not uniform is due to several causes : (1.) The amount of light 

 which passes through any aperture evidently diminishes with the 

 obliquity. (2.) With lenses not free from distortion, the effective 

 aperture itself varies with the angle of incidence. (3.) The amount 

 of reflection from the surfaces of the lenses, and consequently the 

 amount of transmitted light, varies with the angle of incidence. The 

 method of observation above described may be said to fully take into 

 account these three causes of variability in intensity. Then again 

 (4) the light falling on the plate varies inversely as the square of its 



