284 



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



perhaps, be objected that the use of this formula assumes the very 

 thing that the experiments were principally intended to test ; but the 

 objection is evaded, almost if not altogether, when the aperture in 

 the disk is so nearly adjusted to the ideal width that the oblique plate 

 comes to take nearly the same azimuth, for both sets of readings, i.e., 

 with and without the use of the mirror under examination. The use 

 of the formula to allow for a small outstanding difference of obliquities 

 can lead to no appreciable error. If on a first trial a large difference 

 be found, a corrected aperture is calculated with the aid of Pickering's 

 table, and the disk readjusted or replaced. 



A fixed oblique plate has sometimes been used on one or other side 

 of the apparatus in order to effect a rough adjustment of the bright- 

 ness, and to bring the necessary obliquity of the rotating plate to a 

 convenient amount (30° — 60°). This was less trouble than a readjust- 

 ment of the mirrors on the left, with an alteration in the angle of 

 incidence upon the black glass D f . 



In taking an observation the adjustment of the relative brightnesses 

 was facilitated by a device which may now be described. If the 

 attempt be made to secure an absolute match between the two parts 

 of the field in view, a doubt is apt to arise as to whether the disap- 

 pearance of the division line is due to the success of the adjustment 

 or to fatigue of the eyes, leading, as in my case it very rapidly does, to 

 imperfect focussing. This difficulty is less felt when the adjustment 

 is under the immediate control of the observer, who can then satisfy 

 himself of the sensitiveness of his eye by making the necessary 

 displacement ; but in the present experiments (on account of the 

 distance of the telescope) it was convenient to employ an assistant. A 

 glass plate, perpendicular to the path of the light, and attached to a 

 sort of pendulum, was therefore provided on the left hand side, in such 

 a manner that by pulling and letting go a string it could be intro- 

 duced or withdrawn at pleasure. The effect of the plate would be to 

 stop some 8 per cent, of the light, and the adjustment was so made 

 that with glass in the (apparent) right of the field was as much too 

 dark as it was too bright when the glass was out. The difference of 

 brightness, amounting according to the above estimate to 4 per 

 cent., was always fully apparent, and probably no setting more than 

 2 per cent, in error would be allowed to pass, giving, as such would 

 do, a difference of 6 per cent, on the one side and of 2 per cent, upon 

 the other.* Since the auxiliary light is eventually eliminated, it 

 makes no difference, of course, whether we take for the comparison 

 the full light, or the mean of the lights with and without the inter- 

 position of the plate. 



A 2 per cent, error in single settings may lead to a 4 per cent. 



* The accuracy of the settings falls much short of that attained by Professor 

 Rood. 



