﻿Improved Form of Littrow Spectroscope. 141 



indeed, in some cases, be a real advantage, as pointed out by 

 Ames in his paper on the concave grating*. The amount of 

 astigmatism is exceedingly small in an instrument of the 

 dimensions and construction here shown, as the angle between 

 the incident and reflected ray and the geometrical axis is less 

 than one degree. 



In the first trial instrument which was constructed after 

 this plan, the focal length of the mirror was about 175 centim. 

 and its aperture a little less than 6 centim. The prism was 

 of white flint of about the same aperture, with a dispersion 

 of 3J degrees from A to H. Although the apparatus was 

 mostly made of wood, and the adjustments were in conse- 

 quence rather rough and unsatisfactory, the results obtained 

 were even better than I had anticipated. The whole spectrum 

 from the extreme violet to the extreme red was very bright 

 and clean } with the lines very sharply defined. With an eye- 

 piece having a magnifying-power of 70 and a slit-width of 

 *01 millim. (at which width the spectrum was almost too 

 bright for comfort), both the Ni line and the lines at 5892 

 between the D's could be seen, and the doubles in the tail of 

 A were clearly distinguishable f. 



The freedom from diffused light may be judged from the 

 fact that the a and A lines were both clearly seen without the 

 aid of a colour-screen, while with this the lines below A at 

 wave-length 8300 could be seen distinctly without taking 

 any unusual precautions. Better evidence of this point is 

 afforded by photographs of the lower end of the spectrum, 

 which have been taken on an ordinary Cramer Isochromatic 

 plate, whose maximum of sensitiveness lay in the yellowish 

 green. If any considerable amount of diffused light had been 

 present the plates would have been hopelessly fogged by it, 

 as the time of exposure was in some cases nearly an hour. 

 The linear expansion of the spectrum was so small (4*9 centim. 

 from A to D) that some of the detail which was present was 

 masked by reason of the coarseness of the grain of the plate, 

 and a second mirror having a focal length nearly three times 

 as great (470 centim.) was therefore substituted. 



* " Concave Grating in Theory and Practice," Phil. Mag. vol. xxvii. 

 p. 369. 



t If we define the purity of the spectrum by the relation P= — - — R 



1)04"^. 



{Encyc. Brit. Art. Spectroscopy), where D is the width of the slit, 4> the 

 angular value of the collimator aperture, as viewed from the slit, we have, 

 in this case, for yellow light P = 067R, viz. a purity of nearly 70 per 

 cent, of the theoretical resolving-power of the prism was obtained. With 

 the longer-focus collimator a purity of nearly 80 per cent, was reached. 



