38 



Mr. W. H. M. Christie on the 



[Mar. 1, 



It may be remarked that this bright line gives a ready means of adjust- 

 ing the collimator and telescope to focus, as, when this has been done, it 

 should be seen distinctly at the same time as the spectrum- lines. 



The success of this spectroscope has far exceeded all my anticipations. 

 Though the first prism is only 0*75 inch in cross section, and the total 

 length 11 inches, the power is far greater than that of any spectroscope 

 with which I am acquainted, and this is accompanied by remarkable bright- 

 ness in the spectrum. I have compared it carefully with the large spec- 

 troscope of the Royal Observatory, which has a train of ten compound 

 prisms, and find that even with two prisms in the small direct-vision 

 spectroscope the sodium lines are fully as widely separated as, and far 

 better defined than, in the large spectroscope with its full power, whilst 

 their brightness in the direct-vision spectroscope is incomparably greater. 

 With the full train of three prisms this separation is quadrupled nearly, 

 and the sodium lines are seen fully 3° apart with a power of 9 on the 

 viewing-telescope. The compactness of the half-prism spectroscope, the 

 simplicity of its construction, and the ease with which it is manipulated 

 appear to leave little or nothing to be desired. 



I have, however, tried a still more portable form of spectroscope, which 

 for many purposes seems fully equal to the larger form. It is on the plan 

 of the miniature spectroscopes, in which the eye takes the place of the 

 viewing-telescope ; and its general construction will be readily seen from 

 fig. 2, which represents its actual size. 



The prisms turn about centres a, b, being moved by the lever d e f, 

 on which a micrometer- screw m acts as in the larger spectroscope ; but to 

 allow of more motion in the first half-prism the positions of the centres 

 a, b have been altered, which can be done when there are only two prisms 

 in the train, the condition between the lateral displacements still holding. 

 The other arrangements are as before, the prism b being mounted in a 

 small tube so as to be readily withdrawn if large dispersion is not re- 

 quired ; but in place of the viewing-telescope and eyepiece, there is simply 

 an eyeglass, g, adapted to the sight of the observer for distant objects, so 

 that the rays pass through the prisms in a state of parallelism. The 

 bright line formed by the image of the top and bottom of the slit is here 

 seen above and below the spectrum, and measurements are made with 

 great convenience. 



As the whole of the magnifying-power has to be supplied by the prisms, 

 I have increased the angle of the flint to 58° 30', which approaches very 

 closely to the critical angle ; whilst in order to preserve the direct-vision 

 form, the angle of the crown is 120°, giving a magnifying-power of 9 and 

 a dispersion or purity (as the case may be) of 50°, or ten ordinary prisms 

 of 60° for each half -prism. The train of two half-prisms has thus a mag- 

 nifying-power of about 80, and a dispersion or purity of nearly 500°, 

 or 100 ordinary prisms of 60°. 



Small though this spectroscope is (the total length being only four 



