224 Mr. W. B. Cartniol on the Anomnloii.< 



fuchsin-film, ^vhich made the central fringe look just like 

 any other fringe. This plan, however, would have furnished 

 a very decisive solution of the difficulty with regard to the 

 thickness, for by making the plate upon which the fuchsin 

 w^as deposited the back mirror of an interferometer, and com- 

 paring the fringes due to the reflexion from the bare plate 

 with those due to the reflexion from the top surface of the 

 fuchsin, the shift of the central fringe could have been easily 

 determined. 



The photometric measurements were made by means of a 

 Brace spectrophotometer. The methods of adjusting the 

 instrument have been already given by Tuckerman*. 



The extremely great absorption gave rise to some difficulties 

 which do not occur in ordinary photometric work. For 

 instance, a little of the red light (for which fuchsin is trans- 

 parent) reached the eye as stray light, by reflexion in the 

 telescope-tubes and the comparison prism. This, too, in spite 

 of the fact that the telescope-tubes were well blackened and 

 diaphragmed. This small amount of red light was more 

 intense than the screen lio-ht that was beiuo- measured, since 

 the oreen lioht had been cut down a thousand times or more 

 by the absorption of the film. 



This was remedied by using a screen having absorption- 

 bands in the red and the blue, but which transmitted green 

 quite freely. In this way green light alone entered the in- 

 strument when measurements in the green were being made. 



Fig. 5 shows the general arrangement of the apparatus. 

 A beam of sunlight is brought to a focus on each of the 

 totally reflecting prisms p and p^ by means of a split lens S. 

 The prisms reflect the light on to the mirrors m and m', 

 and these direct the light on to the collimator slits. Part 

 of the lioht from the riffht-hand collimator reaches the ob- 

 serving telescope by reflexion from a silvered strip cemented 

 in the comparison prism P. The rest passes above or below 

 the strip, and is lost. On the other hand, of the light which 

 comes from the left-hand telescope, only that which passes 

 above or below the silvered strip reaches the observing tele- 

 scope, with the result that in the eyepiece may be seen a 

 field illuminated from the left-hand collimator, with the ex- 

 ception of a central strip illuminated from the right-hand 

 collimator, and when the intensity of the illumination from 

 both these sources is alike the field appears uniform. This 

 uniformity may be brought about by varying the width of 

 either of the slits S or S'. If the fuchsin-film is placed 

 before the slit S it will cause the centre of the field to seem 

 *- Wied. A7m. xxxi. p. 629 (1888). 



