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[Nature, Vol. LXXXIX. p. 167, 1912.]
THE application of a reflector to pass light back through a prism, c prisms, is usually ascribed to Littrow. Thus Kayser writes (Handbuch de Spectroscopie, Bd. i. p. 513), "Der Erste, der Ruckkehr der Strahlen zu Steigerung der Dispersion verwandte, war Littrow" (0. v. Littrow, Wie'i Ber. XLVJI. ii. pp. 26-32, 1863). But this was certainly not the first use < the method. I learned it myself from Maxwell (Phil. Trans. Vol. CL. p. "7! 1860), who says," The principle of reflecting light, so as to pass twice throug the same prism, was employed by me in an instrument for combining coloui made in 1856, and a reflecting instrument for observing the spectrum hi been constructed by M. Porro."
I have not been able to find the reference to Porro; but it would seei that both Maxwell and Porro antedated Littrow. As to the advantages < the method there can be no doubt. less than from the glass, though still easily visible. In spite of repeated trials with intermediate cleanings, it was difficult to feel sure that the residual effect might not be due to foreign matter, the more as differences could sometimes be detected between various parts of the surface*. Even if the surface could be regarded as clean on immersion, there is no certainty that a capillary film of some sort might not be deposited upon it from the liquid. The cause of the small residual reflexion must remain for the present an open question.