210 Intelligence and Miscellaneous Articles. 



mining the wave-length of certain very brilliant lines like those of 

 lime, strontia, and thallium. It has given me the opportunity of 

 observing that at a high temperature thallium is not monochromatic 

 as had been thought. This fact is not surprising when we consider 

 the great number of lines which soda gives at a high temperature. 



These results are part of a research which I have been pursuing 

 for more than a year in the laboratories of the Ecole Xormale. I 

 have already published, in the Revue des Societes Savants, a Note on 

 the chemical spectra of the alkaline metals, in order to reserve to 

 myself the facility of continuing these researches at pleasure. — 

 Comptes Rendus, January 19, 1563. 



OX A NEW PORM OF SPECTROSCOPE. BY DR. WOLCOTT GIBBS. 



Messrs. J. andW. Grunow, the well-known opticians of New York, 

 have just completed, at my suggestion, a spectroscope involving a new 

 principle, or rather one for the first time applied to instruments of 

 this kind. In this instrument the prism of flint glass has a refract- 

 ing angle of onlv 3 7 ; ; the ravs which diverge from the slit are ren- 

 dered parallel in the usual manner, bv an achromatic lens having the 

 slit in its principal focus. The bundle of rays then falls upon the first 

 surface of the prism at a perpendicular incidence, and of course makes 

 an angle of 37° with the second surface. Under these circumstances 

 the refraction takes place at an angle so near the limiting angle that 

 the refracted rays emerge nearly parallel to the second surface of the 

 prism. The amount of dispersion produced in this manner is very 

 great, while the loss of light, occasioned byrerlexion at the first sur- 

 face in the prisms of 60° placed in the position of least deviation, is 

 avoided. The spectrum thus produced possesses remarkable inten- 

 sity, and the dark lines are seen in countless numbers and with great 

 distinctness. The instrument in this form is sufficient for all chemical 

 purposes ; but it is so constructed as to permit the use of a second 

 prism, by which the length of the spectrum is of course greatly 

 increased. Though the telescopes are only 6 inches in length, with 

 a magnifying power of about 6, the spectrum compares very advan- 

 tageously with that of a large apparatus with telescopes of IS inches 

 focal length and L| inch aperture, and a prism of 60°. I may mention 

 that the centre of the second surface of the prism lies in the vertical 

 axis of the instrument, and also that in a prism of this kind the 

 refracted rays diverge as if from a single radiant point (which is not 

 the case with prisms of the ordinary construction), the angular disper- 

 sion being at the same time much greater. So far as I have been able 

 to find, this form of prism was first employed by Matthiessen. In a 

 lithographed copy of Regnault : s ' Lectures on Optics ' at the College 

 de France in 1848, prisms on this principle, of various forms, are 

 figured and described, together with the spectra produced. These 

 last exhibit an extraordinary extension of the violet end of the spec- 

 trum. A Matthiessen prism of flint glass, in which the first surface 

 is concave so as to admit the addition of a double convex lens of 

 crown glass, appears to be preferable for the spectroscope, in conse- 

 quence of the saving of light. — Silliman's American Journal, Janu- 

 ary 1863. 



