408 Mr. W. Huggins and Dr. W. A. Miller on the 



vatory of Mr. Huggins at Upper Tulse Hill. The object-glass 

 is a very line one, by Alvan Clark of Cambridge, Massachusetts ; 

 the equatorial mounting is by Cooke of York ; and the telescope 

 is carried very smoothly by a clock motion. 



As the linear spectrum of the point of light which a star forms 

 at the focus of the object-glass is too narrow for the observation 

 of the dark lines, it becomes necessary to spread out the image 

 of the star; and to prevent loss of light, it is of importance that 

 this enlargement should be in one direction only; so that the 

 whole of the light received by the object-glass should be concen- 

 trated into a fine line of light as narrow as possible, and having 

 a length not greater than will correspond to that breadth of the 

 spectrum which (when viewed in the apparatus) will be just suffi- 

 cient to enable the eye to distinguish with ease the dark lines by 

 which it may be crossed. No arrangement tried by us has been 

 found more suitable to effect this enlargement in one direction 

 than a cylindrical lens, which was first employed for this purpose 

 by Fraunhofer. In the apparatus by which the spectra described 

 in our "Note" of February 1863 were observed, the cylindrical 

 lens employed was plano-convex, of 0*5 inch focal length. This 

 was placed within the focus of the object-glass and immediately 

 in front of the slit of the collimator. 



The present form of the apparatus is represented in Plate V. 

 figs. 1 and 2, where the cylindrical lens is marked a. This is 

 plano-convex, an inch square, and of about 14 inches focal length. 

 The lens is mounted in an inner tube, b, sliding within the tube 

 c, by which the apparatus is adapted to the eye-end of the tele- 

 scope. The axial direction of the cylindrical surface is placed 

 at right angles to the slit d, and the distance of the lens from 

 the slit within the converging pencils from the object-glass is 

 such as to give exactly the necessary breadth to the spectrum. 



In consequence of the object-glass being over-corrected, the 

 red and, especially, the violet pencils are less spread out than the 

 pencils of intermediate refrangibility ; so that the spectrum, in- 

 stead of having a uniform breadth, becomes slightly narrower at 

 the red end, and tapers off in a greater degree towards the more 

 refrangible extremity*. 



* The experiment was made, of so placing the cylindrical lens that the 

 axial direction of its convex cylindrical surface should be parallel with the 

 direction of the slit. The line of light is in this case formed by the lens ; 

 and the length of this line, corresponding to the visible breadth of the spec- 

 trum, is equal to the diameter of the cone of rays from the object-glass 

 where they fall upon the slit. With this arrangement, the spectrum ap- 

 pears to be spread out, in place of being contracted, at the two extremities. 

 Owing to the large amount of dispersion to which the light is subjected, it 

 was judged unadvisable to weaken still further the already feeble illumina- 

 tion of the extremities of the spectrum ; and in the examination of the 



