384 Professor C. Piazzi Smyth's 



such an untried field in astronomy, that the first successful 

 application of instrumental measure to it will be sure to 

 yield a host of interesting physical facts. Mr Dawes has 

 recently published a method which he has been putting into 

 practice with his equatorial, depending on the contraction of 

 the aperture of the telescope by applying diaphragms, with 

 continually decreasing holes, until the star just vanishes ; the 

 diameter of the hole is then an inverse measure of the bright- 

 ness of the star. 



The method is not new, though perhaps no one else has 

 carried it to the extent to which Mr Dawes has done ; but 

 though, with its instrumental certainty, it would be a great 

 improvement on the present baseless methods which are still 

 permitted to obtain, it can only be regarded as a means of 

 deciding the rough classification of stars into a few ranks of 

 magnitudes, and is not capable of rendering those minute 

 variations of brightness, which may contain the elements 

 of great physical laws still to be discovered, and for this 

 amongst other reasons : 



Variations in magnitude of equal extent at the star must 

 .produce more visible effect to us in the brighter than the 

 fainter stars. Thus, to take an extreme case of a star just 

 visible, and the sun : if they were both to be doubled in 

 brightness, the increase in the former would only be just 

 equal to the smallest unit measurable, or to the probable 

 error of observation ; while in the latter the augmentation 

 would exceed millions of times the smallest appreciable unit 

 of the scale, and the exact extent of the increase, whether 

 exactly double, or differing therefrom by a minute fraction, 

 might be measured to great accuracy. To this result of actual 

 superiority in brightness must be added the similar effect of 

 virtual superiority given by the object-glass of a telescope, 

 increasing the light of the image of the star according to the 

 aperture. 



To gain therefore evidence of minute changes in the light 

 of the stellar orbs, we should employ telescopes of large 

 aperture on large stars. But, according to Mr Dawes' plan, 

 the larger the star the more he reduces the aperture ; in 

 precise proportion as nature offers a larger and larger quan- 



