16 



Mr. E. J. Spitta. 



[Dec. 5, 



of the same diameter this condition of things would not affect the 

 validity of the final results, but it is equally apparent that were the 

 disks of differing diameter the values obtained could not but be 

 seriously affected. Let it be presumed that two such different-sized 

 disks were under consecutive examination, as shown in fig. 2, AB 



Fig. 2. 



F" F' F 



representing the diameter of one and CD that of the other. In the 

 case of AB it is manifest that the extreme limb B would be fainter to 

 all appearance than the opposite edge A, because the light issuing from 

 it has to traverse a portion of the wedge EF, thicker and so more 

 dense than E'F'. On consideration, it is equally obvious that the 

 limiting margin A will be the last to appear as the wedge is made to 

 move from right to left till disappearance takes place, the position 

 technically spoken of as " the point of wedge extinction." But if 

 CD, the larger disk, be illuminated with light of the same initial 

 intensity as fell on AB, it is evident that the point of wedge extinc- 

 tion (technically so-called) for CD at the limb C will not be at the 

 same wedge-reading as in the previous case ; in fact, disappearance 

 would not occur until the portion of the wedge corresponding to the 

 line E'F' had been moved to occupy the position shown by the line 

 E"F". Hence, when ascertaining what is termed the wedge-interval 

 in the direct comparison, say, of the relative intensities of a large and 

 small disk, it is very obvious an error entirely due to the physical 

 nature of a wedge must inevitably result, such error being in direct 

 proportion to the amount of shift required, which depends upon 

 the relative differences in diameter of the disks under observation. 

 Nothing is here said of the difficulties of observation, which are 

 enormously increased by the different apparent intensities of the 

 light at the extremities of the diameter parallel to the length of the 

 wedge, because I merely wish to call attention to the error resulting 

 from the employment of the wedge-form itself. 



To apply a correction under these circumstances was not deemed 

 expedient, even if found to be practically possible ; hence the removal 

 of the source of er~or has been arrived at by devising an instrument of 



