356 P. C. on the Colours that enter into the [Nov. 



medy for this evil may be applied either to the object glass, 

 or to the eye glass ; the images may be brought by the 

 former to positions which will enable us to view them dis- 

 tinctly with the latter ; or the latter may be so modified as 

 to enable us to see the images distinctly without this change 

 in their position ; and, of course, such remedies may be 

 applied to operate jointly. 



Hitherto the attention of opticians has been chiefly 

 directed to the former remedy ; and it appears they have so 

 far succeeded, that the violet and the red images are united 

 in our present achromatic telescopes ; and as these two 

 colours form imaiges upon the retina which with the com- 

 mon telescope differ most in size, we may hope that, when 

 the attention of the artist is directed to a known object, the 

 apparently lesser difficulty presented by the green image 

 will not be long an obstacle to our bringing this noble 

 instrument to perfection. It appears, indeed, that this 

 desirable object has been already attained, by the ingenious 

 combination of a fluid with a solid to form the object lens ; 

 but, from the changeable character of fluids, a correction 

 by means of glasses only, is still a desideratum. 



I have devoted considerable time and attention to the 

 class of coloured fringes connected with this subject, of 

 which I have not met with an explanation in any optical 

 work I have had an opportunity of consulting ; and as it is 

 desirable to form correct views of the principle upon which 

 they make their appearance, not only as it relates to the 

 subject we have adverted to, but, also, as it regards others 

 of almost equal importance, it is my intention to enter more 

 fully into their explanation than would be required to obtain 

 from them the evidence they furnish in support of my prin- 

 cipal object. I feel the more confident that a correct expla- 

 nation has not yet been given of these fringes ; because, it 

 necessarily implies a knowledge of the division of the spec- 

 trum, such as is here set forth ; for no other arrangement 

 will account, satisfactorily, for the different appearances 

 they present. 



Newton has shewn that white light may be re-composed 

 by viewing the spectrum formed by a prism, through another 

 prism similar to it, and held at the same distance from the 

 spectrum, with its refracting angle in the same position. 



