1877.] Magnifying -power of the Half-prism. 35 



From what precedes it will be seen that loss of light is so inseparably 

 associated with dispersion, whether by making it necessary to use a nar- 

 rower pencil or in other ways which are not so easily reduced to law, that 

 there is theoretically but little to choose between different arraugements 

 of prisms, and in particular between a long train of prisms and a short 

 train of half-prisms ; but practically the advantage seems to be altogether 

 with the short train, which is less complicated and more convenient. The 

 difficulties of the adjustment of a long train of prisms are so great that 

 in practice the efficiency secured appears to fall very far short of the 

 theoretical limit, and thus so-called powerful' spectroscopes are a constant 

 source of trouble and disappointment to those who use them. Even if 

 once properly adjusted it is practically impossible to keep them so, and 

 the consequence is that the sodium lines, for example, are seen hardly any 

 better with eighteen or twenty prisms than with four or five. 



The general conclusions arrived at with regard to the best combination 

 of prisms may be summed up as follows : — 



1. The direct- vision prism (as ordinarily used) possesses great advantages 

 over other forms, simple or compound, especially in its correction of the 

 irrationality of dispersion. 



2. This irrationality can, however, be corrected in any form of " half- 

 prism " by keeping the collimator and telescope relatively fixed, whilst 

 the prism only is turned to bring successive parts of the spectrum into 

 the field of view, and thus an angle approaching closely to the critical 

 angle may be advantageously used. 



3. A train composed of three or at most four " half -prisms (direct- 

 vision) magnifying " is the best arrangement for giving wide separation 

 of the lines in the solar spectrum, whilst one or, in some cases, two half- 

 prisms similarly placed offer special advantages in the case of star-spectra. 

 A well-worked slit will be required for examination of the finer lines in 

 the sun's spectrum, and it would be well to use a concave ox " Barlow " 

 lens in the collimator so as to increase its effective focal length. 



4. A train of two or three " half-prisms (direct-vision) diminishing," 

 in combinatign with an eyepiece train of " half-prisms magnifying," is 

 peculiarly adapted for the delineation of the solar prominences, and gene- 

 rally for the examination of bright-line spectra with a continuous back- 

 ground. 



Mr. Hilger, who has entered with the greatest zeal into the practical 

 application of these principles, and to whose skill the success of the idea 

 is largely due, has made a direct-vision spectroscope for the Eoyal Obser- 

 vatory on the following plan (see fig. 1), which has been approved by 

 the Astronomer Royal, and which embodies the above conclusions. The 

 train consists of three half-prisms direct-vision for the red rays, each 

 having an angle of 57° 0' for the flint (sp. gr. 5*0) and 110° for the 

 crown, and giving a dispersion of 20° with a magnifying-power of 3*8. The 

 course of the rays is sufficiently indicated in the diagram, from which it 



d 2 



