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Mr. F. L. 0. Wads worth on 



two latter forms is to be used will be determined by the 

 conditions of use. The second form was the one employed 

 when the wave-comparer was used in connexion with a spec- 

 trometer in experiments on wave-length measurements in the 

 infra-red solar spectrum. In this form the ray is reflected out 

 of the plane of incidence and refraction, and the spectrum is 

 consequently inclined at a small angle to the horizontal, but 

 in this case this was of no disadvantage as it involved only a 

 slight rotation of the observing-slit in order to bring it parallel 

 to the spectral lines. 



Case II. — In the forms so far considered, no special 

 conditions were imposed other than the general conditions 

 of a fixed slit and a fixed observing instrument, it being 

 possible to move the latter to meet the requirements of the 

 particular optical systems which have been described. But 

 in the next application of this system to the great spectro- 

 bolometer of the Astro-physical Observatory, an instrument in 

 which the collimator has a focal length of nearly thirty feet, 

 and the observing telescope a focal length of nearly twelve 

 feet, another condition was imposed, viz. that the observing 

 instrument (in this case the bolometer) should be in the 

 prolongation of the axis of collimation. It was therefore 

 necessary to avoid the lifting of the spectrum above the plane 

 of incidence and refraction, and to place the mirror m at one 

 side of the prism at such an angle to the latter that the direc- 

 tion of the reflected ray was parallel to the line of collimation 

 as required. This throws the mirror-face out of the axis of 

 rotation, and there is therefore a lateral displacement of the 



Fig a 





ray as the system rotates. A plan view of this arrangement 

 is shown in fig. 2. It will be seen that the arrangement of 



