a P. Langley—Determination of Wave-lengths 
ical 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 apparatus, 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, ruled on the surface of a concave mirror of specu- 
lum metal of 1-63 radius of curvature, and exposes a ruled 
surface of 129. 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 apparatus; although, for the 
sake of distinctness, the mechanical devices used to maintain 
the proper arrangements of the parts ure omitted. The rays of 
light, coming from the 800™™ flat mirror of the large siderostat, 
pass across the apparatus, and fall upon a concave speculum OF 
180™" aperture at M, by which at a distance of about i”e they 
are converged to a focus at S,. At this point is a vertical 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®,— 
plate of iron pierced with an aperture only a little larger than 
‘9° 
] A ve 
grating, the slit S,, and the heavy spectro-bolometer, is PIV 
ry massive arm, carrying the 
& 
