90 MR. JOHN BURKE ON THE CHANGE OF 



prevent any light from the vertical surfaces of the cubes A T and A.\ from entering 

 the slits s' and s respectively. We shall suppose that the cubes are illuminated by 

 vertical rays incident perpendicularly to the surfaces A 1( A 2 , A' 1( A' s , i.e., to the plane 

 of the paper. By means of this arrangement we can compare the intensities of the 

 two beams of light coming respectively from the vertical surfaces of the cubes Aj 

 and A'j, opposite to the slits s, s'. None of the light from A x and A 2 is permitted to 

 pass sideways into A.\ and A' 2 , the two sets of blocks being separated by a screen of 

 black paper. On the other hand, the blocks A t and A.\ are separated from A 3 and 

 A' 2 by a sheet of lead glass, which is very opaque to the fluorescence-exciting rays, so 

 that radiation scattered by A 2 and A' 3 cannot be the source of any part of the 

 fluorescence of Aj and A.\, and vice versd. The fluorescence of each block is thus 

 altogether caused by the rays incident perpendicularly to the plane of the paper. 

 If all is right the fluorescent light emitted horizontally from the surfaces , and a\ 

 ought to appear the same, and this is tested by the photometer. 



When this condition is fulfilled three experiments are performed : firstly, to deter- 

 mine the coefficient of transmission of A l or A.\ when fluorescing for those rays 

 which the cube itself gives out by fluorescence ; secondly, to measure its coefficient 

 of absorption when not fluorescing ; and thirdly, to determine independently the 

 ratio of the coefficients, and to compare the value so obtained with that given by the 

 other two results. The last experiment thus serves to verify the first two 

 measurements. 



The (a) Set of Experiments. 



We may call the experiments made to determine the coefficient of transmission 

 during fluorescence the (a) set, and similarly the other two the (/3) and y8/(l -f- a ) sets, 

 in all of which a proper combination of the cubes A^ A' 1( A 2 , A' 2 , is chosen, and the 

 fluorescence-exciting light is allowed to fall upon them, whilst the remaining cube or 

 cubes are screened from its action. 



Thus, if we wish to determine the transmission during fluorescence of the block 

 Aj, we screen A' 2 from the incident rays as in fig. 3, these, it must be remembered, 

 are perpendicular to A 1; A.\, A 2 , A' 2 , that is to the plane of . 



i 1 1 i 1 1 1 i ..i . * t* til i- . ^ 



the paper ; hence the light emitted by the surface a r will 

 be that due to two cubes, and that from a\ to one only. 

 We can compare relative intensities of these by adjust- 



j' 



ing the slits s, s'. The block A' 2 is not removed, as 



some of the rays emerging from c } towards A 2 are reflected back, and these are 

 compensated by a similar reflection from A' 2 . There is no reason to suppose that the 

 reflection would be different according as the reflecting surface is fluorescing or not, 

 because fluorescence is not confined to the surface 



A portion of the light emitted by A 2 is reflected by the double air layers separating 

 A 3 from A 1( but this is included in the coefficient of reflection r. The light emitted 



