﻿140 Mr. F. L. 0. Wads worth on an 



dark as in the ordinary form of spectroscope. This con- 

 struction, therefore, overcomes the main objection to the 

 Littrow form, and enables all the advantages of that form to 

 be realized, while it also possesses, as I shall proceed to point 

 out, certain advantages of its own in the way of greater 

 compactness, better definition, and, finally, greater cheapness 

 of construction. 



As all parts of the instrument save the concave mirror 

 are close together, they may all be placed on a small base, 

 which even for the largest instrument need not be more than 

 8 or 10 in. in diameter, as there are no heavy rotating or 

 overhanging parts, as in the other forms of spectroscope. 



The concave mirror itself may then be placed on a sepa- 

 rate stand and connected with the rest of the apparatus when 

 the instrument is in use, by means of a paper tube or light 

 wooden frame covered with black cloth. Great focal lengths 

 may thus be easily employed without increasing the cost or 

 bulkiness of the instrument, with a corresponding gain in 

 brightness and in definition. Better definition also results 

 from the fact that the number of optical surfaces concerned 

 is less than in any other form of the same dispersive power. 

 Thus in the ordinary Littrow form (one prism) with an 

 achromatic lens there are 8 optical surfaces, and the ray 

 meets 6 of these twice, making 14 reflexions or refractions 

 between the slit and eyepiece. In the ordinary form of 

 equivalent dispersion (two-prism) spectroscope there are 12 

 surfaces and 12 refractions, while in the concave-mirror form 

 there are only 5 surfaces and 8 refractions or reflexions. 

 Moreover, there is no change of focus for different parts of 

 the spectrum, an advantage of some moment when photo- 

 graphs are being taken, and although the concave mirror is 

 astigmatic when used as here shown, this astigmatism is no 

 disadvantage when viewing the image of a slit, and may 



how great care is taken, minute scratches are sure to be made by the 

 polishing pad. 



To obtain such a coat it is necessary :— 1st, that great care be taken 

 in cleaning the glass surface ; 2nd, that pure chemicals be used for the 

 silvering solution; 3rd, that the temperature of the depositing bath 

 be the same as that of the mirror, and preferably below 70 degrees in 

 order that the deposit may proceed slowly and uniformly. I have 

 obtained good results with both the Rochelle-salts process and the 

 Brashear process; but for this purpose prefer the latter, as it gives a 

 very hard film which may be vigorously rubbed with a pad of absorbent 

 cotton while still wet. The former process, however, is, I think, prefer- 

 able for "half silvering," i. e. for obtaining a very even semitransparent 

 film. 



