S. P. Langley on the Selective Absorption of Solar Energy. 173 



special kind of flint glass which has been found nearly trans- 

 parent to all the invisible rays measured. This lens and the 

 slit are fitted into opposite ends of a tube, T, 4^ metres long, 

 held by suitable y's. The beam of rays from the slit, now 

 rendered parallel by the collimator, next falls upon a prism*, 

 P, of the same kind of glass as the lens, supported on a cir- 

 cular adjustable table over the vertical axis of the massive 

 instrument we have called provisionally the spectro-bolometer. 



Whatever the sensibility of the apparatus to heat, it is evi- 

 dent that we cannot accurately map the narrow spectral limits 

 between contiguous heat and cold, unless we can fix their 

 position with exactness. Especially when we consider that 

 these rays are invisible, and that the whole process may be 

 compared to a patient groping in the dark, does the need of 

 an instrument which will record the precise point where a hot 

 or cold line was felt become obvious. This is the object of 

 the spectro-bolometer, which, as well as other apparatus men- 

 tioned here, will be described more particularly in the account 

 of the observations made on Mount Whitney. (It was made 

 by W. Grunow, of New York, from the writer's design.) 



Two long arms, A, A', turn independently about the above- 

 mentioned axis, the angle between them being measured by a 

 graduated circle with two verniers reading to 10". One of 

 these arms is directed toward the slit, and the other toward 

 the spectrum formed by the light on leaving the prism. This 

 latter arm carries at its extremity a concave mirror, M, of 

 98 centim. focus, and on either side of the prism an accurately 

 planed track directed toward the centre of the mirror, on either 

 of which slides a carriage with y's. Into these y's, at B, 

 drops either of two " ebonite " cylinders, one containing the 

 bolometer and the other the ordinary reticule and eyepiece. 

 The bolometer used in the measurements for these maps ex- 

 poses to the spectrum a single vertical strip of platinum, 

 \ millim. wide, covered with lampblack, and placed accurately 

 in the axis of the ebonite cylinder by reversal under a com- 

 pound microscope. The eyepiece also has its cross wires 

 centred in the second cylinder, and serves to examine opti- 

 cally the place which will be occupied by the bolometer-strips 

 when the bolometer-cylinder is in the y's. The optical axis 



* This prism, whose optical properties are in every way excellent, was 

 made by Mr. A. Hilger, of London. Its principal constants are as fol- 

 lows : — size of polished faces, 53 by 49 millim. ; specific gravity, 2*895 ; 

 refracting angle, 62° 34' 43" ; index of refraction for J) line, 1*5798 ; 

 index of refraction for H line, 1'6070. (A rock-salt prism of nearly equal 

 size and great purity, as well as prisms of quartz and spar, have been used 

 to determine the absorption of the glass for each ray, visible and invisible.) 



