322 Mr. Gassiot Description of a Rigid Spectroscope. [June 15, 



are both made of hard white optic crown glass, cut from the same hlock. 

 O is an achromatic ohject-glass of 2 ! 3 inches aperture, and 3 feet focal 

 length. M is a cobweb micrometer eyepiece, having one fixed vertical 

 web, and two which are crossed, moving together ; also a fine rack in the 

 field of view, which serves to register whole turns of the micrometer head 

 (fig. 4). S is a pair of knife-edges. The action is as follows : when any 

 source of light is brought in front of the knife-edges S, some of the rays 

 emitted pass through them, and unchanged through the double diagonal 

 prism D, D', as shown on a large scale in diagram 2. As the object-glass 

 O is placed at its focal distance from the knife-edges, the rays in passing 

 through it are rendered parallel ; on entering and emerging from P', P 

 these rays suffer refraction, and also, if the light be not homogeneous, 

 dispersion. The same effects are produced as the rays enter the first 

 surface of E, and again emerge from it, after being reflected from the 

 further side, which, as has been previously mentioned, is silvered. They 

 now retrace their way through the prisms P and P', the refraction and dis- 

 persion being doubled in this return passage. In this manner a result is 

 obtained equal to that which would be produced by five prisms, if em- 

 ployed in the ordinary manner. 



Repassing through O, this compound lens, which before acted as a 

 collimator, now acts as the object-glass of the telescope T. The cone of 

 rays produced by this lens falls on the prism D D', figs. 1 & 2, and is 

 reflected from the diagonal side, a loss of light determined by the size of 

 the small prism D being experienced ; but as this prism need be but little 

 more than the length and width of the slit formed by the knife-edges, the 

 loss may, practically, be considered unimportant. 



In figs. 1 & 2 the continuous line represents the rays of light in their 

 first passage through the prisms, and the dotted lines the same rays re- 

 turning through the instrument. The image of the slit is viewed, and any 

 change in its position observed, by means of the micrometer eyepiece M. 

 Owing to the power of the instrument, only a very small portion of the 

 spectrum can be seen at once in the field of view. The reflecting prism 

 R is, however, provided with a tangent screw motion, which affords the 

 means of bringing any portion of the spectrum into the field of view that 

 it may be desired to examine. 



Although having to contend with several disadvantages on the score of 

 reflexions not made use of in spectroscopes of the ordinary construction, 

 and which of course cause loss of light and tend to deteriorate the defi- 

 nition, yet it will, I think, be admitted that the performance of the in- 

 strument is satisfactory. 



Mr. W. Huggins has seen two bright lines between the D-lines pro- 

 duced by the flame of a common spirit lamp ; and several persons have 

 seen on different occasions from five to seven lines between the D-lines in 

 the solar spectrum. This is equal to the performance of my large spectro- 

 scope, with which the solar spectrum is now being mapped at Kew Ob- 



