Meteorological and Astronomical Notices. 383 



nences, the light spots, and the dark ones; while, in too 

 many cases, persons have attempted to draw conclusions 

 from the latter only ; probably because they were so much 

 more easily observed than the others. Certainly the bright 

 spots, as well, indeed, as the more characteristic features 

 of the dark ones, require a telescope of large aperture for 

 their clear visibility ; and then comes the difficulty, that no 

 coloured glass can be got to stand the excessive heat any- 

 where near the focus. To overcome this obstacle Mr Dawes 

 has proposed to diminish the aperture of the field bar of the 

 telescope, in proportion to the size of the object-glass ; the 

 effect of which will be, that while the small part of the sun 

 then under view will be formed by all the illuminating power 

 of the object-glass, the total quantity of heat coming through 

 the eyepiece, will be diminished in proportion to the contrac- 

 tion of the field bar. With very large telescopes, perhaps, 

 the stoppage of the heat by the said field bar, and its accu- 

 mulation inside the telescope, might be found to be objection- 

 able, but it does not seem to produce any bad effects in object- 

 glasses of 6 or 7 inches in diameter. 



Variations in the Brightness of Double Stars. — Amongst 

 the multifarious species of variation of brightness amongst 

 stars, Mr Nasmyth has procured some cases of alleged 

 variation in the brightness of double stars occurring simul- 

 taneously in both members. This circumstance he looks 

 on as decidedly favourable to his view of the origin of solar 

 and stellar light, viz., the physical action of the surface of 

 those orbs on the material of light already existing in space. 

 On the supposition of this material existing in different quan- 

 tities in various parts of space, combined with the progressive 

 motion of the star itself, he was able to explain the varia- 

 tion in brightness of a single star, and the discovery that 

 two stars drifting together through space vary simultaneously, 

 must be looked on as much increasing the probability of 

 the explanation, though it is not the only one that can be 

 rendered ; and the observations on which the proof of such 

 variation is founded are by no means so certain or trustworthy 

 as could be wished. 



Measurement of the Brightness of the Stars. — This is still 



