﻿Improved Form of Littrow Spectroscope. 137 



in the preceding calculations. And it does not seem probable 

 that a better knowledge of the varying conductivity of rocks, 

 or of their contractibility on cooling, would give such altered 

 values to the constants of the problem as to bring up the 

 calculated amount of elevations so as to approach the actual. 

 Dr. Murray estimates the mean elevation of the land of the 

 globe at 1947 feet above the sea, and the mean depth of the 

 ocean at 12,456 feet*, whereas, according to my estimate, 

 the mean height of the elevations which would be caused by 

 the compression of a solid earth, initially supposed at 7000° F., 

 would be only 6^ feet |. 



If we make the supposition that the crust rests on a liquid 

 substratum, we shall still have a level of no strain J, and 

 although the amount of the corrugations formed will in this 

 case be somewhat greater, it will still fall far short of that 

 which the " positive teachings of geology " require us to 

 account for. 



XII. An Improved Form of Littrow Spectroscope. 

 By F. L. 0. Wadsworth§. 



[Plate VI.] 



IT is rather remarkable that in the development of the 

 prismatic spectroscope there have been no important 

 modifications in the general design of the instrument as first 

 used by Newton, progress having been marked mainly in 

 the mechanical improvements of the various parts. Various 

 modifications have, it is true, been proposed from time to 

 time, but none of them has stood the test of usage, except 

 perhaps the form which was first proposed by Littrow in 

 1862 ||, and has since been modified and improved by Young 

 and Lockyer% Browning**, Grubbft, Bracket J J, and 

 others. 



In this form, as is well known, the rays from the slit, after 

 being rendered parallel by a collimating-lens, pass through 

 the prism or train of prisms and fall normally upon a plane 



* * Scottish Geographical Magazine/ June 1888, vol. iv. 



t ' Physics, &c.,' 2nd ed. p. 103. 



\ ' Physics, &c.,' Appendix, chap, xxviii. 



§ Communicated by the Author. 



|| American Journal of Science, 2nd series, vol. xxxv. 



^| Schellen, Spectralanalyse, vol. i. p. 231. 



** Ibid. p. 237. 



ft Monthly Notices of the Royal Astronomical Society, vol. xxxi. p. 36. 



\\ American Journal of Science, 3rd series, vol. xxiv. p. 60. 



