i6 W. B. Cartmel 



The same trouble was experienced in the measurement of the 

 retardations with the interferometer, but when the thickness was 

 definitely known a comiparison with a film sufficiently thin to 

 show bands all through the spectrum cleared up this point, for 

 with the very thin film the retardation was never so much as a 

 whole wave-length, for reasons already mentioned. The matter 

 might have been cleared up by viewing the fringes of white light 

 in the usual way and observing the shift of the central fringe 

 produced by the fuchsin, but this was found impracticable on 

 account of the selective absorption of the fuchsin film, which 

 made the central fringe look just like any other fringe. This 

 plan would have, however, furnished a very decisive solution of 

 the difficulty with regard to the thickness, for by making the plate 

 upon which the fuchsin was deposited the back mirror of an 

 interferometer, and comparing the fringes due to the reflection 

 from the bare plate with those due to the reflection 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 transparent) reached 

 the eye as stray light by reflection in the telescope tubes and th€ 

 comparison prism. This, too, in spite of the fact that the tele- 

 scope tubes were well blackened and diaphragmed. This small 

 amount of red light was more intense than the green light that 

 was being measured, since the green light 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 instrument when mea- 

 surements in the green were being made. 



Fig. 4 shows the general arrangement of the apparatus. A 

 beam of sunlight is brought to a focus on each of the totally re- 



^Astrophysical Jourfial , p. 145. Oct., 1902. 



ii6 



