ABSORPTION PRODUCED BY FLUORESCENCE. 97 



The screen is so arranged as to admit of being worked by the observer, by means of 

 handles, whilst he is otherwise engaged in taking observations through the eyepiece, 

 but the mechanism by which this is effected is not shown in the figure, which is a 

 sketch of the arrangement adapted chiefly to photographic work. In the eye obser- 

 vation an optical bench was used, being a suitable stand for the apparatus. 



To test the accuracy attained and also the uniformity of the illumination, or rather 

 its symmetry, we may give the result of thirty observations. These observations 

 were made in the earlier stages of the experiments, when but little experience had 

 been acquired. If the illumination had been uniform, the intensity of the fluorescent 

 light should have appeared the same when the slits were of equal width. 



Fifteen readings of the left slit, when the right was fixed at 8, give 7 '8 7 as the 

 mean of the readings, the probable error of a single observation being 0'54, and the 

 probable error from the mean 0'135. Similarly the other fifteen observations, when 

 the left slit was fixed at 8 and the right slit movable, gave the mean of the readings 

 of the right slit to be 7 '73, the probable error of a single observation being 0'53, and 

 that from the mean 0*133. It may be remarked that in this particular experiment 

 the readings of each observation were taken with an opening slit, and the apparent 

 equality was obtained before the real equality existed, that is that in both sets of 

 observations the mean of the readings was somewhat less than what it should have 

 been ; but the process of reversing eliminates this. Then the readings to the two 

 sets of observations are 7'87 0'135 and 7'73 0'133. The ratio of the illumination 



fr.oy / A .1 OK\ 



of the cubes on the right to that of those on the left is - - ( 1 ~z~j^ ) in the first 



o / / n.i QQ\ 



set of observations, and 77^.3 / I 1 .-, ) in the second, if we neglect the square 



lid I \ {'tO / 



/0-133 



of 



7-73, 



The ratios of the illumination of the cubes on the right to that of those on 

 the left, as given by the two sets of observations, are (0'984 0*017) and 

 (T035 0'017). The mean of these two ratios is TOOO. The probable error in the 

 ratio is 0'017 in either case. Hence the probable error from the mean, since the 

 two determinations of the ratio of the two illuminations are quite independent, is 



=- = 0'012. And the probable error of thirty observations is about 1 per cent. 



from the mean. 



In the detection of the change of absorption, an error of 20 per cent, will obliterate 

 the effect. 



In tabulating the experiments, the following method has been adopted. The 

 cubes are indicated by the letters A 1( A 2> A' l; A' 3> the arrangement being 

 as shown in the accompanying figure. 



The intensities compared are those of the rays proceeding from the 

 faces a i and a\. 



VOL. CXCI. A. O 



