VOL. XI.] PHILOSOPHICAL TRANSACTIONS. 335 



ton, it being so agreeable to the received laws of refractions. And indeed the ob • 

 servations of these two learned persons, as to this particular, are easily reconcile- 

 able to each other, and both to truth; Mr. Newton contending only for the 

 length of the image, transverse to the axis of theprism, in a very clear day ; and Mr. 

 Line only nnaintaining the excess of breadth, parallel to the same axis, while the 

 sun is in a bright cloud. Though as to what is further delivered by Mr. Newton, 

 and opposed by Mr. Line, namely, that the length of the coloured image was 

 five times the diameter of its breadth; I never yet have found the excess above 

 thrice the diameter, or at most 3 i, while the refractions on both sides the prism 

 were equal. So much as to the matter of fact. 



Now as to Mr. Newton's theory of light and colours, I confess, his neat set 

 of very ingenious and natural inferences, was to me on the first perusal a strong 

 conjecture in favour of his new doctrine. But since several experiments of re- 

 fractions remain still untouched by him, I conceived a further search into them 

 would be very proper, in order to a better discovery of the truth of his assertion. 

 For, accordingly as they are found either agreeing with, or disagreeing from, his 

 new theory, they must needs much strengthen, or wholly overthrow the same. 

 The experiments I pitched upon for this purpose, are as follow : 



1. Having frequently observed that the form of objects viewed in the micro- 

 scope, or rather of the microscope itself, consists almost in an indivisible point, 

 I concluded that two very small pieces of silk, the one scarlet, the other violet 

 colour, placed near together, should, according to Mr. Newton's theory, appear 

 in the microscope in a very different degree of brightness, in regard that their 

 unequal refrangibility must cause the scarlet rays or species to over-reach the 

 retina, while placed in the due focus of the violet ones, and consequently occa- 

 sion a sensible confusion in the vision of the former, one and the same point of 

 the scarlet object affecting several nerves in the retina. Yet on frequent trials 

 I have not been able to perceive any inequality in this point. 



1. The second experiment was made in water. I took a brass ruler, and 

 fastening to it several pieces of silk, red, yellow, green, blue, and violet, 1 placed 

 it at the bottom of a square vessel of water : then I retired from the vessel so far 

 as not to be able to see the ruler and coloured silks otherwise than by help of 

 the refracted ray. Now, did Mr. Newton's doctrine hold, I conceived I should 

 not see all the said colours in a straight line with the ruler, as the unequal refran- 

 gibility of different rays must needs displace some more than others. Yet in 

 effect, on many trials, I constantly found them in as straight a line as the bare 

 ruler had appeared in. 3. To confirm this experiment, I added a second refrac- 

 tion to the former of the water, by placing my prism so as to receive perpendi- 

 cularly the refracted species of the silk and ruler; by which only the emergent 



