96 MR. JOHN BURKE ON THE CHANGE OF 



whole amount of light scattered has been found to be inappreciable (see p. 93), and 

 on this account alone the portion now under consideration may be neglected. In the 

 second place, if the error were appreciable, it would tend to diminish rather than 

 to increase the apparent difference between the absorptions before and during 

 fluorescence respectively. For the scattered light would add to the total light 

 emitted from a L , a\, and would therefore tend to reduce the effect. 



The length of the horizontal slit tt being about 2 centime., and its width 2 millims., 

 the image was 6 centims. by 6 millims., for the focal length was 4'5 centims., and 

 the focal distances 6 centims. and 18 centims. respectively. By using a lens of 

 shorter focal length, however, an image of tt can be formed on the horizontal slit of 

 a spectroscope placed so that the refraction takes place in a vertical plane, and 

 observations can be made of the absorption in different parts of the spectrum. This, 

 however, is by no means an easy matter to accomplish, and we have confined ourselves 

 to the light taken as a whole. 



With reference to the illumination, it was thought at first that one of the 

 electrodes might possibly always be brighter than the other. Accordingly a plan 

 was adopted of continually reversing the current in the primary coil by means of a 

 wheel attached to the commutator of the coil, which was made to rotate by means 

 of a motor. Unfortunately the exposures required in the photographic work were 

 thereby greatly increased. The experiments, however, show that the average 

 illumination obtained from each electrode is, for all purposes with which we are 

 concerned, practically the same. 



EYE OBSERVATIONS. 



The accuracy of these observations has been found to depend, to a very large 

 extent, upon experience on the part of the observer in dealing with the relative 

 intensity of the lights placed side by side. It is, of course, a familiar fact in 

 photometry that it is very difficult to form a judgment as to the equality of two 

 illuminated surfaces when the illumination is either very feeble or very powerful ; 

 and accordingly this is a source of considerable inaccuracy in such determinations. 

 It is not desirable in either case to examine the relative intensity attentively for any 

 length of time, as it is obviously possible, by viewing two feebly illuminated surfaces 

 side by side for a sufficiently long time, to imagine that they are almost equal when 

 really they are not so, and this remark appears to be particularly applicable to such 

 cases in which the two colours are of slightly different tints. Instances have arisen 

 in which it has besn impossible to arrive at any result whatever as to the relative 

 intensity. In such cases, however, the difference of tint was conspicuously marked. 



It is preferable, when the observations are made by eye, to make a large number 

 of measurements quickly, and to take the means of these first impressions, noting in 

 turn the maximum and minimum widths of the slit, and then reversing the system of 

 screens which serve to shelter the combination of the cubes from the exciting rays. 



