680 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



be not done, deceptions will take place, r. 4 remarkable points : very distinct. 

 o. Better illuminated : very distinct, y. Still better illuminated : very distinct : 

 The points all over the field of view are coloured ; some green ; some red ; some 

 yellow; and some white, encircled with black about them. Between yellow and 

 green is the maximum of illumination. Extremely distinct, g. As well illumi- 

 nated as the yellow : very distinct, b. Much inferior in illumination : very distinct. 

 i. Badly illuminated : distinct, v. Very badly illuminated : I can hardly see the 

 object at all. 



Exper. 7. The nail again, at 8 feet from the prism, r. I attended to 2 bright 

 points, with faint ones between them almost all the points in the field of view 

 are red : very distinct, o. I see all the points better : they are red, green, yellow, 

 and whitish, with black about them : very distinct, y. I see better : more bright 

 points, and more faint ones : the points are of various colours : very distinct, g. 

 I see as well : the points are mostly green, and brightish-green, inclining to white: 

 very distinct, b. Much worse illuminated : very distinct. 1. Badly illuminated: 

 very distinct, v. There is hardly any illumination. 



Exper. 8. The nail again, at 9 feet 6 inches from the prism, by way of having 

 the rays better separated, r. Badly illuminated : the bright points are very dis- 

 tinct, o. Much better illuminated : the bright points very distinct, y. Still 

 better illuminated : all points extremely distinct, g. As well illuminated, and 

 equally distinct, b. Badly illuminated : the bright points are distinct ; but the 

 others are not so. 1. Very badly illuminated : I do not see distinctly ; but I believe 

 it to be for want of light, v. So badly illuminated that I cannot see the object ; 

 or at least but barely perceive that it exists. 



Exper. 9. Black paper at 8 feet from the prism, r. The object is hardly visible: 

 I can only see a few faint points, o. I see several bright points, and many faint 

 ones. y. Numberless bright and small faint points. Between yellow and green, 

 is the maximum of illumination, g. the same as the yellow, b. Very indiffer- 

 ently illuminated ; but not so bad as in the red rays. 1. I cannot see the object. 

 v. Totally invisible. 



From these observations, which agree uncommonly well, with respect to the 

 illuminating power assigned to each colour, we may conclude, that the red-making 

 rays are very far from having it in any eminent degree. The orange possess more 

 of it than the red ; and the yellow rays illuminate objects still more perfectly. 

 The maximum of illumination lies in the brightest yellow, or palest green. The 

 green itself is nearly equally bright with the yellow; but, from the full deep green, 

 the illuminating power decreases very sensibly. That of the blue is nearly on a par 

 with that of the red; the indigo has much less than the blue; and the violet is very 

 deficient. 



With regard to the principle of distinctness, there appears to be no deficiency 

 in any one of the colours. In the violet rays, for instance, some of the experi- 

 ments mention that I saw badly; but this is to be understood only with respect to 



