1835.] Composition of White Light. 363 



yellow colour to one of the glasses of a telescope, would, in 

 many cases, be attended with advantage. The opacity of 

 colouring matter used in giving this tint to glass, is so de- 

 cided to violet light, that a piece of it in my possession, the 

 colour of which is confined to one surface of the glass, upon 

 which it forms a mere film, appears to exclude it almost 

 entirely; while it freely transmits the green and red. No 

 other colour can be so completely excluded, by any means 

 with which I am acquainted, without a considerable depth 

 of the medium, and a consequent loss of the two colours 

 which it is the object to preserve. 



Sir David Brewster, in his treatise on Optics,* suggested 

 such an improvement under an impression that yellow is a 

 simple colour, and, consequently, that it would render the 

 telescope monochromatic ; but although I do not consider 

 this opinion correct, I think the advantages anticipated by 

 Sir David Brewster will not be lessened by it ; as it appears 

 two, instead of one, of the most luminous colours will be 

 admitted ; and the images formed by them, it is probable, 

 may be corrected with greater ease and precision than when, 

 as at present, we have three colours to contend with. 



One of the great defects of the telescope, it is well known, 

 arises from the want of perfect coincidence in the indepen- 

 dent images formed by the light of different colours. The 

 colours which surround the collected images are not the 

 cause af this defect, but one of its consequences, and, there- 

 fore, an indication of its existence ; but their removal, 

 supposing the colourless part to remain unaltered, would 

 not produce the slightest advantage. 



Let us suppose that we have three pictures so precisely 

 alike that if they were placed upon each other the different 

 objects represented in them would exactly correspond; let 

 us further suppose these pictures to be transparent, and 

 that white objects are represented on one picture red, on 

 another green, and on the other violet. Now, it is evident 

 that if it were possible to form such an arrangement, and 

 that the light transmitted to the eye from the three pictures 



* If we could obtain a solid or a fluid which would absorb all the other rays of 

 the spectrum but the yellow, with as little loss as there is in red glasses, a telescope 

 of the preceding construction would answer for day objects, and for all the pur- 

 poses of astronomy. — Treatise on Optics, Lardner's Cyclopedia p. 368. 

 VOL. II. 2 A 



