288 Di\ William Huggins [Feb. 6, 



by clockwork will remain invariably pointed to any star, notwith- 

 standing that the rapidly rotating earth is carrying the telescope and 

 the observer round with it. This clock motion is one of exceptional 

 excellence, due to the inventive skill of Mr. Grubb, being furnished 

 with a secondary control by means of a pendulum in electrical con- 

 nection with a standard clock. But even these instrumental arrange- 

 ments, although of exceptional excellence, were not delicate enough. 

 It was found necessary to supplement them with a method of con- 

 tinuous supervision and control by hand. 



In this diagram (Fig. 2) you have represented a portion of the 

 reflecting telescope, which is of the Cassegrain form. The small 

 mirror was removed, and the spectrum apparatus accurately adjusted 



Fig. 2. 



by its sliding base plate, so that the slit was brought precisely to the 

 principal focus of the large speculum. Now over this slit is placed 

 a highly polished silver plate c, with a narrow opening rather larger 

 than the slit. 



The next point was to fix on the side of the telescope a small 

 mirror d, by which artificial yellow light could be thrown upon the 

 plate. One point further. The great speculum has a central hole ; 

 now behind this, in place of the usual eyepiece, is fixed a small 

 Galilean telescope or opera-glass. 



Now if the observer directs the telescope to a star, and then looks 

 into this small telescope, he sees before him the silver plate and the 

 slit within the opening by means of the artificial illumination, and 

 also at the same time the star's image as a bright point somewhere on 

 the plate. It is then easily within the observer's power to bring the 

 star's image exactly upon any desired part of the slit. In the figure 

 at 2, Fig. 1, you have represented what the observer sees. The star's 



