252 



defined. The ring is still a mere bar lying across the equator : it 

 was very well shown, as were also three satellites. When I applied 

 the power 400, the whole field was strewed with harsh light. 



10. y Virginia. — This interesting star, though now so exceedingly 

 close, was made double with 250, and very well shown ; but with 4U0 

 there was great tremor and irradiation, so that the discs were often 

 confused into one. 



Saturday y March 30, 1833. — After a continuance of bad weather 

 for several days, it cleared off a little ; but in the mean time I had 

 missed the favourable phase of the moon, for which I had been wait- 

 ing. I therefore closed my examinations with the following one : 



The Sun. — From the extreme volatility of the sulphuret of carbon, 

 I was fearful of its expansion, and therefore had not yet turned the 

 telescope upon the sun, lett the condensation of the solar rays, at the 

 place where they traverse the fluid, should prove too much for the 

 lenses. But on mentioning this apprehension to Professor Barlow, 

 that gentleman assured me that an exposure of from five to ten mi- 

 nutes could do no mischief. I therefore this day reduced the aperture 

 to three inches, and directed the instrument to the solar disc, when, 

 sweeping over the luminary for about three minutes, I found the sur- 

 face was quite clear of spots. On turning from it, I drew out the 

 eye-tube, and looking at the fluid, perceived that the bubble was con- 

 siderably diminished, but not so much as I had expected. This was 

 the only time that 1 exposed the telescope to great heat. 



These are the only experiments I have been able to make • and 

 the season of the year, together with the inefficiency of the apparatus, 

 have certainly prevented me from assigning exact limits to the per- 

 formance of this telescope. Still, as I had immediate reference to one 

 of the best refractors extant, I may add the following conclusions, 

 premising, that I have not constantly noted down the performance of 

 the latter upon each test, because my end was to pronounce upon the 

 fluid object-glass. I should also observe, that the magnifying powers 

 of both the instruments were equally matched, and their apertures 

 were generally proportioned to nearly six inches : the eyepieces 

 were thus : 



Fluid refractor 90 150 250 400 



Flint-glass ditto 93 157 240 416 



From the result of my observations, it has struck me that this in- 

 genious principle has strong claims to consideration for its valuable 

 optical powers, but that, in the present stage, it is more adapted for 

 stars than for planets ; and should the application of it be tried on a 

 larger scale, it might be made with sufficient illumination to examine 

 the high-class nebulae ; a branch of practical astronomy which is now 

 nearly shut against refractors. The defining power does not strike me 

 as being so good as the light, nor does the achromatism seem to be 

 perfect. Yet 1 should mention the want of focal and mechanical ar- 

 rangement 3 that the only adjustment I had for distinct vision was by 

 the hand, with the sliding eyepiece tube ; and that slight derange- 

 ments might be occasioned by the mounting and dismounting of the 

 great tube, however carefully it was attended to. 



