152 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



millimeter or half that juimber) from which the, diffracted invisible ray fell on the bolometer at a 

 measured angle with the grating. By the use of the known formnla {ns \=sin i-\-sin r), connecting 

 the angle of dittractiou with the wave-length, the wavelength was then found. 



Several determinations were thus made of wave-lengths in the upper part of the infra-red, 

 where the heat is relatively great; but, though the definition of the Eutherfued grating was 

 admirable, it was not large enough to supply sufficient heat to enable measures in the lower infra- 

 red to be made with confidence. 



In May, 1882, I had the good fortune to secure one of the very large concave gratings, then 

 newly constructed by Professor Eowland, and which he was kind enough to make for me of a 

 very short focus, so as to give a specially hot spectrum. After many essays, during which a great 

 number of mechanical and optical arrangements for getting rid of the superposed spectra were 

 tried with unsatisfactory results, it became clear that, for this large and concave grating, it was 

 necessary to let the ray fall first on it and then on the prism, thus making the wave-length the 

 known and the deviation the unknown quantity. 



In the use of this form of grating, the slit is placed In the circumference of a circle whose 

 diameter is equal to the radius of curvature of the grating and which touches its surface. The 

 spectra are then formed, without the need of collimator, observing telescope, or any further appa- 

 ratus, all lying upon the circumference of the circle which contains the slit. The grating which 

 was employed contains 18,050 lines, 142 to the millimeter (3,G10 per inch), ruled on the surface of a 

 concave mirror of speculum metal of 1™.63 (61 inches) radius of curvature, and exposes a ruled 

 surface of 129'"" (20 square inches). By this large surface a spectrum is produced sufficiently hot, 

 even in its lower wave-lengths, to affect the bolometer strips after the various reflections and 

 absorptions to which the heat is necessarily subjected in passing through the apparatus. 



Figure 1 illustrates the means finally adopted and the course of the rays through the appa- 

 ratus; although, for the sake of distinctness, the mechanical devices used to maintain the proper 

 arrangements of the parts are omitted. The rays of light, coming from the 12-inch flat mirror of 

 the large siderostat, pass across the apparatus, and fall upon a 7-inch concave speculum at M, by 

 which at a distance of about 5 feet they are converged to a focus at Si. At this point is a ver- 

 tical slit, adjustable to any desired width by a double screw, which moves both jaws at once, so as 

 to keep the center always in the same place. This slit is protected from the great heat by a plate 

 of iron pierced with an aperture only a little larger than the slit when open to the usual width. 

 Beyond Si the rays diverge and fall upon the concave grating, G. Directly opposite the grating 

 is a second slit, S2, also double acting, and the apparatus is so arranged, that the two slits, Si, S,, 

 and the grating, G, always lie upon the circumference of a circle whose diameter is 64 inches; and 

 therefore in whatever manner the slits may be placed, the light coming through Si forms a sharp 

 spectrum upon S2. A very massive arm, carrying the grating, the slit S^, and the heavy spectro- 

 bolometer, is pivoted at the center of the circle, so that the relative positions of these parts are 

 unchanged. The slit S2 is automatically kept diametrically opposite the grating, and on the normal 

 to its center. 



The slit S2 is the slit of the spectro-bolometer, provided with' the same attachments as when 

 used for mapping the visible spectrum (except that it is now fitted with simple collimating and 

 objective lenses of the same special kind of diathermanous glass as the prism, instead of its own 

 concave mirror). Its eye-piece and the bolometer are interchangeable. 



By means of the eye-piece and graduated circle, the deviation, and consequently the refractive 

 index of the rays passing through the slit can be determined, if they are visible. If they are 

 invisible, their exact wave-length is known by a simple ocular observation of the visible ones, on 



