1886.] Light reflected at nearly Perpendicular Incidence. 283 



can indeed secure that the lights seen in immediate juxtaposition come 

 from the same part of the ground glass, but a corresponding perfection 

 of adjustment does not apply to other parts of the field. If we 

 suppose ourselves to be looking through the telescope at the ground 

 glass, the part seen to the right of the division line really lies to the 

 right on the ground glass. On the other side there is a distinction, 

 according to the two positions of the shifting mirror. When the 

 revolving disk is in use, the circumstances on the right hand side of 

 the apparatus correspond to those on the left, and thus the part of the 

 field seen to the left of the division line really comes from the left on 

 the ground glass. The ground glass is thus seen much as if it were 

 looked at directly, in spite of the separation of the light into two parts 

 following distinct courses. On the other hand, when the additional 

 reflector (under examination) is brought into play, there is another 

 inversion, and the part of the ground glass seen to the left comes 

 really from the right of the central line. In this case, therefore, it is 

 the same part of the ground glass which is seen in both final mirrors. 

 The distinction here pointed out would be of no consequence if the 

 field were absolutely uniform, or if it were possible to compare the 

 parts seen in immediate juxtaposition, without regard to the parts a 

 little further removed. But if the original field vary slightly in 

 brightness from right to left, it will be a ^question how far the eye 

 would select for the match continuity of brightness across the division 

 line, or how far it would demand equality in the average brightnesses 

 of the two parts presented. 



It now remains to describe certain accessories. During the obser- 

 vations it is necessary to have some means of varying the relative 

 brightnesses of the two parts of the field without removing the 

 mirrors or altering the width of the slit in the revolving disk. For 

 this purpose a plate of glass (L), capable of rotation about a vertical 

 axis, was introduced into the path of the light on the right hand side 

 of the apparatus (between the second and third reflections). As the 

 angle of incidence upon this plate increases, a greater proportion of 

 the light is reflected and thrown away, and a less proportion is trans- 

 mitted to the eye. 



The observation consists in -varying the azimuth of this plate until 

 the match is satisfactory, after which the obliquity of the plate is 

 measured. The transmission by the plate at the measured obliquity 

 can then be found approximately from Fresnel's formula.* It may, 



* A convenient table is given by Pickering ('Phil. Mag.,' vol. 47, 1874, p. 129). 

 If A be the proportion of light reflected at a single surface, the transmis- 

 sion through a transparent plate is given by (1 — A)" + (1 — A) 2 A 2 + (1 — A) 2 A 4 + 

 . . . . = (1 - A) / (1 + A) . The whole reflection is thus 2 A/ (1 + A) , from which 

 Pickering's table is calculated. An erratum may be noted. For 65° the reflection 

 should be 39*6, not 38*4, the value of A being supposed to be sin 2 (0 — 9^1 sin 2 (9 + 1 ) 3 

 while sin 9 = 155 sin 9 X . There are some other minor inaccuracies. 



