1869.] Cn ike Formation and Phenomena of Clouds. 317 



absorption is remarkable, and apparently cannot be explained by the 

 colour of the glass. 



No. 8 is of 60°; its \x for E=T620. It is free from colour, and an 

 evident improvement on the earlier ones. 



No. 9, a prism of 90°, was given to the author by Dr. Lloyd for a small 

 mirror in the Newtonian form of the Armagh 15-inch reflector ; its \x for 

 E=l-6188. 



No. 10, of 90°, was obtained by the late Lord Rosse to be similarly used 

 in his 3-feet Newtonian ; its li for E = l*5321. 



No. 11, of 60°, obtained at Munich in 1837. For these measures the 

 ends were polished flat ; its li for E = 1*6405. 



These three show considerable progress, and an object-glass made of 

 such materials would have a great power of transmission, though much 

 behind the following. 



No. 12 is of 90°. Its glass is from Chance ; its p for E = 1'6216. 



No. 13 is a cylinder 2*2 inches in diameter, and 4*3 long, which Mr. Grubb 

 obtained from Messrs. Chance for these measures ; its fx for E = 1*5200. 



No. 14 is a cylinder got at the same time, 2*1 inches in diameter and 

 4*4 long ; its li for E = 1*6126 ; the ends of both are polished flat, and they 

 are of wonderful transparency. 



If, as there is good ground for hoping, Messrs. Chance shall succeed in 

 manufacturing large disks of the same perfection as these two cylinders, 

 the author's comparison of the achromatic and the reflector must be 

 considerably modified. 



Assuming w=*02, he calculates that the aperture of an achromatic, of 

 focal length equal to 18 times the aperture, equivalent to a 4-feet Newto- 

 nian, is 35*435 inches. This aperture would be diminished if the process 

 of cementing were found applicable to lenses of such magnitude. 



The author concludes with suggesting that, as very slight variations in the 

 manufacture of glass seem to make great changes in its absorptive power, 

 it would be prudent to examine the value of n in the disks intended for 

 lenses of any importance. This could be done by polishing a couple of 

 facets on their edges, and need not involve the sacrifice of many minutes. 



II. " Note on the Formation and Phenomena of Clouds." By John 

 Tyndall, LL.D.. F.R.S. Received January 25, 1869. 



It is well known that when a receiver filled with ordinary undried air is 

 exhausted, a cloudiness, due to the precipitation of the aqueous vapour 

 diffused in the air, is produced by the first few strokes of the pump. It is, 

 as might be expected, possible to produce clouds in this way with the 

 vapours of other liquids than water. 



In the course of the experiments on the chemical action of light which 

 have been already communicated in abstract to the Royal Society, I had 

 frequent occasion to observe the precipitation of such clouds in the experi- 

 mental tubes employed • indeed several days at a time have been devoted 



