Absorption on the Resolving Power of Prism Trains. 367 



Table II. 



i ' 



-R/3. 



25,000 



50,000 



75,000 



100,000 



125,000 



150,000 



200,000 



300,000 



400,000 



500,000 



600,000 



800,000 



1000,000 



II. 



III. 



IY. 



24375 







44680 



48765 



49275 



59152 







67750 



90830 



95970 



71675 







72420 



122220 



137070 



68880 



142460 



169820 



56760 



157140 



217830 



46440 



152720 



239440 



38750 



141850 



244200 



33060 



129420 



239160 



25440 



107200 



217280 





90100 



192700 



considerably larger than can be used to best advantage. The 



Young spectroscope has a theoretical resolving-power r of 



nearly 300000 in the same region of the spectrum, but as 



will be seen from the Table or the Curve the effect of 



absorption cuts down the practical resolving-power to about 



57000, actually less than that which an instrument of only 



one-fourth the size would possess. 



It is at once evident from these results that if high-power 



prism spectroscopes are to be used in the investigation of 



the photographic region of the spectrum, the use of the extra 



dense flint glass, so commonly employed in the past, must 



be avoided, not only on the score of light-efficiency, but, as 



now appears, on the score of photographic resolving-power 



and purity as well. The use of lighter Hint reduces the 



theoretical resolving-power, r, of any given prism-train by 



cln 

 decreasing the value of the dispersion-coefficient — ; but this 



may be easily and even advantageously compensated by 

 increasing the refracting angle cj> of the prisms. The 

 advantages of this latter construction on the score both of 

 light-efficiency and economy of material were pointed out by 

 the writer several years ago*, and it has been adopted in all 

 the large spectroscopes recently constructed at Allegheny f. 



* Astrophysical Journal, vol. ii. p. 264 (1895). 



f Among- others may be mentioned the spectroscope of the Phila- 

 delphia Observatory, the spectroscope of the Lowell Observatory, and 

 the spectroheliograph of the Philadelphia Observatory (just completed). 

 Prof. Lord, of the Emerson McMillen Observatory, has also recently 

 replaced the battery of dense Hint prisma first constructed for his star 

 spectroscope with one of light Hint; and I believe Dr. Gill, of the Cape 

 Observatory, is likewise considering' this change. 



