102 DR L. BECKER ON THE SOLAR SPECTRUM. 



reference, and Angstrom himself ascribes the S-band to the same medium which produces 

 the A, B, and a-groups, it is not clear whether Professor Smyth arrived at this conclusion 

 from his own observations, or from those of the Swedish physicist. However, since 

 Angstrom's band extends much further to the refrangible side, we are inclined to think 

 that the note represents Professor Smyth's opinion. 



The water- vapour band (t), between b and F, is described by Angstrom as very 

 strong in summer. It is the same which Mr Maxwell Hall has utilised as a rain- 

 indicator at Jamaica. 



3. The Instrument. 



The optical part of the instrument is the same as has been used at Dun Echt Observa- 

 tory for several years. The sun's rays, after reflection by the heliostat, fall on an object- 

 glass, of 6 inches aperture and 7 feet focal length, which forms an image of the sun on 

 the slit attached to the collimator. By two endless cords the observer can correct the 

 position of the heliostat without going outside the hut. The slit is formed by two plates 

 of platinum with both jaws opening simultaneously by the motion of a screw. By 

 a rack and pinion the slit can be brought into the focus of the collimating lens. The 

 latter has a free aperture of 4 inches and a focal length of 4 feet. Two feet in front of 

 it the Rowland grating stands on the faceplate of the recording apparatus. It is fixed 

 in a brass frame with a T footpiece, with levelling screws at the ends. The Rowland 

 grating — a present from the Johns Hopkins University at Baltimore to the Earl of 

 Crawford's Observatory — contains 14,438 lines to every inch, ruled on speculum metal, 

 its ruled surface being 5 "5 by 3 '5 inches. Although there is a slight difference in the 

 focus of the spectra on either side of the normal, we are convinced that the irregularities 

 in ruling which cause this defect have been without influence on this work. This is 

 satisfactorily shown by the fact, that close double lines, which were separated by 

 Professor Piazzi Smyth with similar optical appliances, were found to be double and 

 well defined at Dun Echt. Moreover, a great number of faint lines, never recorded 

 before, were observed on both sides of the normal of the grating, their reality being often 

 abundantly established by their increased intensity in a low sun. 



The diffracted rays are received by the 4-inch object-glass of the viewing telescope, 

 of which the focal length is 4 ft. 11 in. There is a filar micrometer provided with two 

 cross wires inclined 45° to the horizon. Their intersection serves as the zero point. 

 An eye-piece, with a magnifying power of 120 diameters, was employed on all occasions. 

 The viewing telescope forms an angle of 25° with the collimator. Each is supported on 

 a separate concrete pier. 



The recording apparatus consists of two distinct parts, one for magnifying the 

 angular motion of the grating, and the other for recording the corresponding arc on a 

 broad fillet of paper. The grating stands on a plate attached to the same vertical axis 

 as a 6-inch worm-wheel (A) of 180 teeth. This wheel is turned by a tangent-screw (a) on 



