260 PHILOSOPHICAL TRANSACTIONS. [aNNO 1675-6. 



the horns than the other parts of the moon. At the first observation, or a little 

 sooner, the horns were parallel to the horizon. Then also Porphy rites and the 

 great black lake were equally out of the shadow, viz. about the length of Me- 

 rseotis. The shadow went out near the upper Hyperborean lake, the penumbra 

 remaining, which exhibited the eclipse to the naked eye as far as 4^ 1 5^"*. But 

 the limb left by the eclipse did not recover the brightness of the other limb till 

 4^ 28"^, or later. The times of the phases were corrected by altitudes of Arctu- 

 rus and the bright star in the Crown; to the taking of which altitudes I applied 

 myself at intervals, when clouds came over the moon, those stars sometimes 

 shining out very bright in the other part of the heavens. 



j4 Letter of Mr. Franc. Linus, from Liege, the 15th of Feb. 1675, iV. S. being 

 a Reply to the Letter printed in Number 110, by way of Answer to a former 

 Letter of the same Mr. Linus, concerning Mr. Isaac Newton's Theory of 

 ■ Light and Colours. N° 121, p. 499. 



Hon. Sir. — In yours of Dec. 17, you say, I may rest assured, first, that the 

 experiment was made in clear days ; 2dly. that the prism was placed close to the 

 hole, so that the light had no room to diverge; and 3dly. that the image was 

 not parallel, as I conjectured, but transverse to the axis of the prism. Truly, 

 sir, if these assertions be admitted, they do indeed directly cut off what I said 

 of Mr. Newton's being deceived by a bright cloud. But if we compare them 

 with Mr. Newton's relation of the experiment in the Phil. Trans. N° 80, it 

 will evidently appear, they cannot be admitted, as being directly contrary to 

 what is there delivered. For there he tells us, the ends of the coloured image 

 he saw on the opposite wall, near five times as long as broad, " seemed to be 

 semicircular." Now these semicircular ends are never seen in a clear day, as 

 experience shows. From whence it follows, against the first assertion, that the 

 experiment was not made in a clear day. Neither are those semicircular ends 

 ever seen when the prism is placed close to the hole : which contradicts the se- 

 cond assertion. Neither are they ever seen when the image is transverse to the 

 length or axis of the prism : which directly opposes the third assertion. But if, 

 in any of these three cases, the image be made so much longer than broad, as 

 easily it may by turning the prism a little about its axis, near five times as long 

 as broad, than the one end thereof will run out into a sharp cone or pyramis, 

 like the flame of a candle, and the other into a cone somewhat more blunt, 

 both which are far from seeming semicircular; whereas, if the image be made 

 not in a clear day, but with a bright cloud, and the prism not placed close to 

 the hole, but at a competent distance from the same, as you see it placed in the 

 scheme of the experiment in N° 84, then these semicircular ends always appear 



