280 



Lord Rayleigh. On the Intensity of [Nov. 18, 



metal surface plate was laid down upon the table as the support for 

 this stand, and carefully levelled. Under these circumstances the 

 mirror if vertical in one position will remain vertical even though 

 displaced ; and this remains true, even though the feet of the stand do 

 not rest immediately upon the plate, but upon small flat buttons of metal 

 of uniform thickness, and perforated with equal holes, by which the 

 feet of the stand are guided to definite positions. When the adjust- 

 ments are complete, these buttons are fastened to the surface plate by 

 dropping cement round their edges. 



The position F may now be chosen at convenience, and without any 

 particular care except in the levelling. The central ray, as fixed by 

 the diaphragms, should fall near the middle of the surface. The other 

 positions would also be somewhat arbitrary were it not for the neces- 

 sity of securing the same angle of incidence and reflection in the two 

 positions. To assist in this a small frame of brass wire is provided, 

 carrying two pointers, and so arranged that it can always be placed in 

 an absolutely definite position with respect to the mirror. By means 

 of hooks it makes two contacts with the back, and two with the upper 

 edge of the mirror. Of the other two contacts required to make up 

 the necessary six, one is with the lower part of the face of the mirror, 

 and the other with one of its vertical edges. Of the pointers, one (in 

 the path of the incident ray) leads upwards, and the other (in the 

 path of the reflected ray) leads downwards. By bending them suit- 

 ably their extremities may be brought into the path of the central 

 ray, so that when the eye is placed in such a position (H) as to see 

 the central point A in the middle of the (apparently elliptical) aper- 

 ture B,* this central point is just enclosed between the barely meet- 

 ing pointers. By so choosing the second position, D, that this 

 condition is again satisfied to an eye looking along ED, we secure 

 not only the same angle of reflection but the use (for the central ray) 

 of the same part of the glass. In making the adjustment we may 

 first bring the pointer on which the incident ray strikes into the 

 already determined line CD, and then rotating the apparatus about 

 the vertical through this point, bring the second pointer to coincide 

 with the reflected ray. 



We have now to consider how to fix the position of E, the re- 

 flector under examination. Replacing the shifting mirror into its 

 first position, F, we mark the line of the central reflected ray, EH, 

 by needles, standing up from the table and as far apart as convenient. 

 Transferring the shifting mirror to the position D, we have so to place 

 E that the reflected ray shall coincide with the same line as before. 

 For this purpose not only must the azimuth of E be correct, but its 

 plane must be brought into the intersection of the already determined 



* Auxiliary lighting, with a candle or otherwise, is sometimes necessary in order 

 to see these apertures properly. 



