106 



Lieut. J. Herschel's Account of the [Nov. 19, 



approach each other equally. Half the length of this slit may be obscured 

 by the intervention of a right-angled prism, which reflects a side light 

 through it if required. The converging rays from the object-glass falling 

 on the slit are admitted, while those which do not are stopped. The former 

 diverging again, as though from a luminous line, emerge from the next lens 

 and fall on the prism as parallel rays, are independently refracted and dis- 

 persed in traversing it, and after emergence are again condensed, but not 

 reunited, by the object-glass in the small telescope composing the second 

 of the above-mentioned tubes, and, forming a spectrum in its focus, are 

 viewed as such by an eyepiece. 



Means of measurement. — The direction of emergence defines the position 

 in the spectrum ; and the difference of direction is measured by the change 

 of direction of the small telescope necessary to receive the several refracted 

 rays directly. This change of direction is effected and measured by a tan- 

 gent screw, whose complete revolutions are indicated by the march of a 

 graduated scale (attached to the telescope-arm) over a circle marked on 

 the circumference of the divided cylindrical head of the screw. The posi- 

 tion of the centre of motion of the telescope-arms, it should be said, 

 though optically unimportant, is practically within the prism. By the help 

 of a reading-lens the revolutions, and tenths and hundredths of a revolution, 

 can be easily read off by a very slight movement of the eye from the eye- 

 piece. 



New graduated Scale for micrometer measures. — A mistake having oc- 

 curred in graduating the scale, I substituted one of my own making. As 

 I was fortunate in this, I may venture to describe how it was effected. The 

 graduation required was too fine for any ink lines I could make ; I there- 

 fore varnished a piece of card, and drew fine lines at the proper intervals 

 on the shellac-coating with a sharp blade ; and applying a little ink, these 

 were instantly rendered visible* I then cut the card across the lines and 

 glued the scale so formed over the old one with varnish, giving the whole a 

 dash of varnish for the sake of protection. When dry I was gratified to 

 find the graduation correspond well with the revolutions ; for it was rather 

 a delicate job, and I did not succeed without failures. 



6. Graduated Scale in the Field of View. 

 The third tube was intended to present in the field of view of the tele- 

 scope last described, by external reflection off the second surface of the 

 prism, an illuminated image of a photographed scale placed at one end of 

 the tube, in the principal focus of a lens at the other. The tube carries a 

 small moveable mirror outside. Upon this mirror was intended to be thrown 

 the light of a small lamp, held n position by a bent arm projecting from 

 the prism-chamber. I am sorry to say that this ingenious contrivance 

 proved, in my hands, more unsatisfactory than perhaps it should have done. 

 As in not using it I departed from the letter of my instructions, I am in a 

 measure bound to explain my reasons for discarding it. 



