336 PHILOSOPHICAL TRANSACTIONS. [aNNO I676, 



species suffered a second refraction. But still with equal success, as to their^ 

 appearing in a straight line, to the eye placed behind the prism. 4. To these 

 two refractions I further added a third, by receiving the coloured species 

 obliquely on the prism ; by which both incident and emergent species suffered 

 their respective refractions. But still with the same success as formerly, as to the 

 straight line they appeared in. 



For further assurance in this experiment, lest prepossession, occasioned from 

 previous knowledge of the silks situation in a straight line, might possibly pre- 

 judice the judgment of the eye, as sometimes I have observed to happen to the 

 judgment the eye passes on the distance of objects, I called into the room some 

 unconcerned persons, wholly ignorant what the experiment aimed at ; and de- 

 manding whether they saw not the coloured silks and ruler in a crooked line ? 

 they answered in the negative. 



5. The next experiment I made in uncompounded Colours, (as Mr. Newton 

 terms them, prop. 5 and 13) as follows; Having cast two coloured images on 

 the wall, so as the scarlet colour of the one fell in a straight line, parallel to the 

 horizon, with the violet of the other; I then looked on both through another 

 prism, and found them still appear in a straight line parallel to the horizon, as 

 thev had formerly done to the naked eye. Now according to Mr. Newton's 

 assertion of different refrangibility in different rays, I conceive the violet rays 

 should suffer a greater refraction in the prism at the eye, than the scarlet ones, 

 and consequently both colours should not appear in a straight Hne parallel to the 

 horizon. 



6. Another experiment I made, in order to some further discovery of that 

 surprising phenomenon of the coloured image, which occasioned Mr. Newton's 

 ingenious theory of light and colours, as also his excellent invention of the 

 reflecting telescope and microscope. Having then sometimes suspected, that 

 not only the direct sun-beams, but also other extraneous light, might possibly 

 influence tlie coloured spectrum, I hoped to discover the truth of this suspicion 

 by means of the sun-spots, made to appear in the coloured image by placing a 

 telescope behind the prism. But my endeavours proving ineffectual herein, by 

 reason of some intervening difficulties, I thought at length of a more feasible 

 method in order to the designed discovery, as in the following experiment. 



I fastened a very white paper circle, about an inch in diameter, on my window- 

 shutter ; and viewing it through my prism, I found a coloured image painted 

 by it on my retina, answerable in almost all respects to the image of the sun- 

 beams on the wall, especially when the paper circle was indifferently well illumi- 

 nated. This image indeed appeared contrary to the former as to the situation of 

 colours, that is, the scarlet appearing above, the violet below, though but faint. 



