1886.] Light reflected at nearly Perpendicular Incidence. 279 



caused to revolve the view is cut off and blackness substituted for 

 about nineteen-twentieths of the whole time. When the speed ex- 

 ceeds about twenty-five revolutions per second, there is no perceptible 

 flickering, and the light is seen with a simple diminution of bright- 

 ness. The idea of the method is so to adjust the angular opening 

 that the effects of the glass under test and of the disk shall be equal. 



The two brightnesses last considered can only be seen successively. 

 They are separately tested with a comparison light, reflected at B' 

 from the same primary beam. After reflection at a silvered mirror, 

 C, and then at a blackened glass, D', this light falls upon a silver- 

 on-glass speculum at H', and passes thence to the observing telescope 

 at I. In setting up the apparatus a control over the brightness of 

 the comparison light is obtained by varying the angle of incidence 

 upon D'. 



In order that the line of division between the two fields, as seen 

 from I, may be quite sharp, it is necessary that the final reflector H 

 be a speculum. To obtain a reflecting surface, perfect up to the very 

 edge, a piece of silvered glass is carefully cut (on the glass side) with 

 a diamond. If the operation is properly performed, the silver is left 

 undisturbed, and when the plate is inclined, as at H, no part of the 

 glass substratum is visible. 



In adjusting the apparatus the object aimed at is to cause the 

 central ray, ABB', issuing* from A on the ground glass, to assume 

 ultimately the position HI, whether it proceed by the course on the 

 left, or by either of the alternative courses on the right. As may be 

 supposed, this is more easily said than done. All the reflectors 

 require to be adjusted so as to be perpendicular to the plane in which 

 the central ray is to travel. This plane is conveniently taken hori- 

 zontal, so that every reflector has to be vertical.* 



The central ray is defined by diaphragms at A, B, B', points in the 

 same horizontal straight line. At A the light is admitted through a 

 small aperture only. At B, B' the holes are cut in thin cardboard 

 screens held in definite positions up to the glasses. It was found 

 convenient to have them rather large — about half an inch in diameter. 

 In setting up the apparatus the glasses B, B', C are readily put into 

 position, accuracy being required only in the levelling. The line CDF 

 is now defined, and the next step is the more difficult one of fixing the 

 two positions for the stand carrying the mirror D. This stand is 

 (like all the others) provided with levelling screws, but these must 

 not be used in passing from the one position to the other. A heavy 



* The levelling of the reflectors was effected with the aid of the straight edge of 

 a long board, adjusted until it coincided with the prolongation of its image. My 

 assistant, Mr. Gordon, is expert at this adjustment of the edge to perpendicularity 

 with the reflecting surface. The vertically of the latter is then tested by the 

 application of a spirit-level to the edge of the board. 



VOL. XLI. U 



