VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 670 



bright constellation of thousands of luminous points, scattered over its whole 

 extent, as far as the field of the microscope could take it in. Their light was that 

 of the illuminating colour, but differed considerably in brightness; some of the 

 points being dim and faint, while others were luminous and brilliant. The 

 brightest of them also admitted of a little variation in their colour, or rather in 

 the intensity of the same colour; for, in the centre of some of the most brilliant 

 of these lucid appearances, their light had more vivacity, and seemed to deviate 

 from the illuminating tint towards whiteness, while on and near the circumference 

 it appeared to take a deeper hue. An object so well divided by nature, into very 

 minute and differently-arranged points, on which the attention might be fixed, in 

 order to ascertain whether they would be equally distinct in all colours, and whe- 

 ther their number would be increased or diminished by different degrees of illumi- 

 nation, was exactly what I wanted ; nor could I think it less remarkable, that all 

 the other objects I had fixed on, besides many more which have been examined, 

 such as copper, tin, silver, &c. presented themselves nearly with the same appear- 

 ance. In the brass, which had been turned in a lathe, the luminous points were 

 arranged in furrows; and in tin they were remarkably beautiful. The result of 

 the examination of my objects was as follows. 



Exper. 2. Red paper. In the red rays, I view a bright point near an accidental 

 black spot in the paper, which serves me as a mark; and I notice the space be- 

 tween the point and the spot : it contains several faint points. In the orange rays, 

 I see better. The bright point I now perceive is double. In the yellow rays, I see 

 the object still better. In the green rays, full as well as before. In the blue rays, 

 very well. In the indigo rays, not quite so well as in the blue. In the violet rays, 

 very imperfectly. 



Exper. 3. Green paper. Red. I fix my attention on many faint points, in 

 a space between 2 bright double points. Orange ; I see those faint points better. 

 Yellow ; still better. Green ; as well as before : I see remarkably well. Blue ; 

 less bright, but very distinct. Indigo; not well. Violet; bad. 



Exper. 4. A piece of very clean turned brass. 



r. I remark several faint luminous points between 1 bright ones. The colour 

 of the brass makes the red rays appear like orange, o. I see better, but the 

 orange colour is likewise different from what it ought to be ; however, this is not 

 at present the object of my investigation, y. I see still better, g. I see full as 

 well as before, b. I do not see so well now. i. I cannot see well. v. Bad. 



Exper. 5. A nail. r. I remark 1 bright points, and some faint ones. o. 

 Brighter than before; and more points visible: very distinct, y. Much brighter 

 than before; and more points and lines visible: very distinct, g. Full as bright: 

 and as many points visible : very distinct, b. Much less bright ; very distinct, i. 

 Still less bright : very distinct, v. Much less bright again : very distinct. 



Exper. 6. I viewed a guinea, at Q feet 6 inches from the prism ; and adjusted 

 the place of the object in the several rays, by the shadow of the guinea : If this 



