278 PHILOSOPHICAL TRANSACTIONS. [aNNO I676. 



from his friends, if my declaration of my meaning satisfy not, I shall note some 

 further passages in my letter, whereby they may see how I was to be under- 

 stood from the beginning, as to the aforesaid three circumstances. 



For the day; I express every where that the experiment was tried in the sun's 

 light, and in N° 80, that the breadth of the image by measure answered to the 

 sun's diameter ; but because it is pretended, I was imposed upon, I would ask, 

 what the experiment as it is advanced to that which I called the experimentum 

 crucis, can have to do with a cloudy day ? for if the experimentum crucis, which 

 is that which I depend on, can have nothing to do with a cloudy day, then is it 

 to no purpose to talk of a cloudy day in the first experiment, which does but 

 lead on to that. But if this satisfy not, let the Transactions, N° 83, be con 

 suited; for there I tell you how, by applying a lens to the prism, the straight 

 edges of the oblong image became more distinct than they would have been 

 without the lens; a circumstance which cannot happen in Mr. Linus's case 

 of a bright cloud. 



For the position of the prism ; I tell you, N° 80, that it was placed at the 

 sun's entrance into the chamber, and I directed to make a hole in the shutter, 

 and there place the prism, and in the next page I say again, that the prism ABC 

 is to be set close by the hole F of the window EG; and accordingly represent 

 it close in the figure. Also in another page I tell you, that the distance of the 

 image from the hole or prism was 22 feet; which is as much as to say, that the 

 prism, suppose that side of it next the hole, was as far from the image as the 

 hole itself was, and consequently that the prism and hole were contiguous. 

 Also in the next page, where instead of a window shutter I made use of a hole in 

 a loose board, I tell you expressly, that I placed the board close behind the prism. 

 All these passages are in my very first letter about colours, and who therefore 

 would imagine that any one that had read that letter, should so much as suspect 

 that I placed the prism, I say not at so great a distance as Mr. Linus supposes, 

 but at any distance worth considering. 



Lastly, for the position of the image, it is represented transverse to the axis 

 of the prism in the figures N° 80, N" 83, and N* 85. And in N° 88, where I 

 made use of two cross prisms, I tell you expressly, that the image was cross to 

 both of them, at an angle of 45 degrees. The calculations also, N" 80, are not 

 to be understood without supposing the image cross. Nor are my notions about 

 different refrangibility otherwise intelligible ; for, in Mr. Linus's supposition, 

 the rays that go to the two ends of the image are equally refracted. So for 

 colours; the red, according to my description, falls at one end of the image, and 

 the blue at the other ; which cannot happen but in a transverse image. The 

 same position is also demonstrable from what I said in N° 80, about turning the 



