Dr Goring on Monochromatic Light, %c. 59 



what circumstances, they are achromatic. I propose to show 

 that an engyscope cannot be made achromatic in any other 

 manner than as a telescope is, and that the new achromatics 

 are consequently not superfluous. 



The angle of aperture of the erecting eye-pieces of spy-glasses 

 is usually very small, not exceeding five or six degrees; the 

 chromatic aberration, therefore, of their objective part, is near- 

 ly insensible to vulgar eyes. Moreover, it is well known that 

 all working opticians are in the habit of over-correcting the 

 colour in the object-glasses of those telescopes which are ex- 

 pressly made for viewing terrestrial objects. It is supposed by 

 some that the predominance of the blue and purple rays, which 

 is occasioned by this over-correction, tends to exhibit terrestrial 

 objects better than they would be seen if the instruments were 

 perfectly achromatic,— an absurdity which 1 shall not take 

 the trouble of refuting. 



The real manner in which a beneficial operation is produced 

 by this over-correction for colour, is by its effect on the eye- 

 piece, which it greatly ameliorates and improves, by neutra- 

 lizing the red and orange light, which would otherwise greatly 

 preponderate in it, so that the totality of the instrument may 

 be considered sufficiently achromatic for practical purposes; 

 nevertheless, under certain circumstances, the uncorrected co- 

 lour of the eye-piece renders itself very manifest ; as, for ex- 

 ample, when we view some dark object, such as the rigging of 

 a vessel or a weathercock, against the light of a clear sky. For 

 by placing the object at the edge of the field of view, while we 

 look obliquely at it from the opposite side, so as to catch an 

 eccentrical pencil, the uncorrected colour is instantly felt. 



Now, as I am contending for the principle of achromatism, 

 I shall proceed to show, that all erecting eye-pieces of the or- 

 dinary construction, though they may, under certain circum- 

 stances, and in a practical point of view, pass for achromatic, 

 yet have in good earnest no achromaticity about them ; and 

 of this I think any man may convince himself by the follow- 

 ing experiment -.-There is a small circular disk of braes w.th 

 a hole in it, placed between the two bottom glasses of all erect- 

 ing eye-pieces, which, when the telescope IB in action, serves 

 only "to exclude the false light whirl, is reflected by the Ion- 



