1q6 philosophical transactions. [anno 1797. 



unnecessary to be more particular, because the time of my observation in the 

 beginning of the transit on the disc, I find was only taken down in whole minutes 

 of the clock. The end however is more accurately determined, by the observation 

 which was made 45 s after the immersion; when a part of the disc, equal to about 

 -J- of the diameter of the satellite, is said to be visible. It seems that observations 

 of this kind, made with very good telescopes, charged with high powers, are 

 capable of great precision. For the remark that a margin of Jupiter, equal to 

 about -J- of the diameter of the satellite, became visible in 45 s of time, adds 

 great support to the accuracy of the observation of the foregoing 4 minutes ; and 

 at all events it is evidently proved, from the whole of the entrance on the disc, 

 that the diameter of the satellite is less, by one half at least, than what from the 

 result of the measures of former observers it has been supposed to be. 



A method has also been used, of deducing the diameter of the satellites from 

 the time they employ to immerge into the shadow of the planet ; but this must be 

 very fallacious, and ought not to be used. 



I should not pass unnoticed the apparent magnitude of the satellites. The ex- 

 pressions that have been given of them may be collected into the following narrow 



compass: 1,4,2 4;1 3 — , 4 ; 1 — 2 4,2,1 3 4;1;2 1,4,2 



3 — 2,1,4 3 2—1,4 1T24 . — 2 1 ; 2 — 4 3 1,2 2—1. 



From which we may conclude, that the 3d satellite is considerably larger than 

 any of the rest; that the 1st is a little larger than the<2d, and nearly of the size 

 of the 4th; and that the 2d is a little smaller than the 1st and 4th, or the smallest 

 of them all. 



XVI. Further Experiments and Observations on the Affections and Properties of 

 Light. By Henry Brougham, Jun. Esq. p. 352. 



I am first to unfold a new, and I think curious property of light, that may be 

 indeed reckoned fourfold, as it holds, like the rest, equally with respect to refrac- 

 tion, reflexion, inflexion, and deflexion; thus preserving entire the same beautiful 

 analogy in these 4 operations, which we have hitherto remarked. I shall then 

 consider several phenomena connected either with this, or with the properties be- 

 fore described, and of which they afford some striking confirmations. 



I. Observation 1. The sun shining strongly into my darkened chamber, I 

 placed, at a small hole in the window-shutter, a prism with its refracting angle (of 

 65°) upwards, so that the spectrum was cast on a chart placed at right angles to 

 the incident rays, and 4 feet from the prism. In the rays, parallel to the chart, 

 and 2 feet from it, I placed a pin, whose diameter was ^ of an inch, and fixed it 

 so, that the axis of its shadow on the spectrum might be parallel to the sides of 

 the spectrum. A set of images by reflexion was formed similar to those described 

 for 179^j all inclining to the violet; but what I chiefly attended to at present was 

 their shape. I had always observed that the part formed out of the red-making 



