USEFUL NOTICES. 117 



being set very steadily. But upon this I rested my left hand, 

 between the finger and thumb of which I held the eye end 

 of the glass. In this situation, the glass being truly adjusted 

 to distinct vision, I conld observe the star, and by gently 

 and rapidly striking the tube with the fingers of the other 

 hand, I caused the image of the star to dance in the field of 

 view, and describe the same kind of luminous line as is seen 

 when a lighted coal is whirled about. The star was thus 

 made to describe by each blow a curve returning into itself ; 

 but so contorted and irregular that no two successive curves 

 were coincident with each other. The strokes were about 

 ten in a second of time, and the curves were beautifully and The rays were 

 distinctly tinged with different colours in their successive parts beautifully 

 thro' different lengths : but it seemed at a medium that each vivid, 

 of these vivid colours might occupy afeout one-third part or 

 less of the whole curve, and upon my recollection those most 

 predominant were greenish blue, steel blue, and maroon or an in- 

 tense copper colour. The light from Sirius therefore as it arrived and varied 



at the eye was by extremely sudden variations distinctly chane- thirty time* 

 . . . . . , . . . , *t , in a second- 



ed in its colour, at least thirty times in one second. No theory 



deducible from the known properties of the atmosphere, as 



an interposed medium, has yet presented itself to my mind, 



in a shape worthy of notice. 



In the collection last quoted of Rochon, p. 380, he observes, Correspondent 

 that the scintillation of the fixed stars is an obstacle to mea- fzc } Wlth » 

 suring their diameters, and that when the light of Sirius was 

 refracted into colours by a prism, it had no scintillation across 

 the spectrum. As far as may relate to the apparent diameters 

 of the fixed stars, the observations of Herschel do not seem 

 to support the deduction of Rochon j but his fact appears to 

 correspond with mine. 



3. Advantage of upsetting or pressing in the borders of plates 

 offlintglass to make the concave lens in achromatic combinations. 



The same Abbe Rochon p. 372, remarks that the triple object J»rge achr©. 

 lenses of Dollond of 3£ inches aperture, produce an effect matIC Iense ' 

 equal to that which it seems ought to be obtained from the 

 lenses of 30 or 40 feet, made by Campani. Rut that in 

 making achromatic lenses of longer focus, the plates of glass cannot be 

 being blown, are too thin to be worked without bending and made for want 

 spoiling the figures. All the cast glass he tried was found to g i TW ,H * 



N 



