36 keport — 1859. 



On an Improvement in the Heliometei'. 

 By Norman Pogson, Director of the Hartwell Observatory. 

 The purpose of this communication is to suggest what I conceive to be a great 

 addition to the power of any kind of micrometer used for measuring long distances 

 on the double-image principle. It is therefore especially applicable to heliometers, 

 and has indeed occurred to me chiefly from familiarity with the defects which have 

 hitherto rendered this costly but magnificent instrument a comparative failure. It 

 is well known to practical astronomers that the contact between two stars, however 

 skilfully made, is a very unsatisfactory observation, even when the objects are pretty 

 equal. But when one is a large bright star and the other a faint one, the difficulty 

 and uncertainty amount to impossibility ; for the faint star is invariably obliterated 

 on approaching within two or three seconds of its superior. The alternative is then 

 to diminish the aperture of that half of the object-glass through which the brighter 

 star is viewed ; but here again arises another evil ; the disc is enlarged by diffraction, 

 the value of the scale sensibly changed, and definition materially injured. Hence 

 parallax determinations of first magnitude stars, such as Arcturus and a Lyrse, 

 cannot be satisfactorily made ; but when the object is a double star, as, for instance, 

 61 Cygni or Castor, the comparison star can be brought between the components of 

 the double star, and a most exquisitely perfect and comfortable measure obtained. 

 Now, from having used the rock-crystal prism micrometer when residing at Oxford 

 last year, — then kindly lent me, together with a five-foot telescope of surpassing excel- 

 lence, by Dr. Lee, — the idea occurred to me of introducing a prism, or achromatized 

 wedge of rock-crystal, into the heliometer, so as to double the image of the brighter 

 star. By this means the dubious contact would be dispensed with ; for the fainter 

 object, by being brought midway between the two images of the bright star, would 

 be precisely similar to the present easy observation of 61 Cygni previously referred 

 to. The prism could be of such a constant angle as to separate the two images to a 

 convenient distance ; not too far, so as to render the estimation of distance difficult, 

 but just wide enough to prevent the obliteration of a faint comparison star, before 

 named as one of the evils to be avoided. The prism rather improves the appearance 

 of a bright star than otherwise; and as the images are doubled, of course half the 

 light of each is lost, equivalent to a considerable reduction of the aperture, thus 

 obviating the third objection alluded to at starting. Armed with this addition to its 

 strength, and taking the precaution never to observe on bad nights, when the 

 atmosphere will not permit the use of powers from three hundred upwards — fori 

 hold it as an absurdity to attempt to investigate tenths of a second of arc with any- 

 thing less — the heliometer is doubtless yet destined to realize the highest expectations 

 ever raised, as to its efficiency for grappling with that most minutely intricate and 

 vastly important research, viz. the parallax of the fixed stars ! 



On three Variable Stars, R and S Ursa Major is, and U Geminorum, as 

 observed consecutively for six years*. By Norman Pogson, Director of 

 the Hartwell Observatory. (Communicated by Dr. Lee.) 



[With a Plate.] 



The periodical variation in brilliancy of certain fixed stars has now been known to 

 astronomers for more than two centuries. The fact of simple change, apart from 

 periodicity, has been recognized and recorded for nearly two thousand years ; and 

 while every other celestial phenomenon has been explained and reduced to intelligible 

 methods of calculation, based upon theories as incontrovertible as the events they 

 foretell in future or account for in past times, these changes of light and colour 

 remain enshrouded in mystery, and their prediction as purely empirical as was that 

 of eclipses by the Chaldeans of old, aided by their renowned Saros, or eclipse-period 

 of 223 lunations. 



It is not, however, for want of due thought and attention from the eminent 

 astronomers of the past and present that such a reproach attaches to any branch of 

 their science. Commencing with Fabricius, who first drew attention to the dis- 



* This paper was illustrated by large diagrams of the light curves of the above three vari- 

 able stars, covering an area of more than sixty square feet. The portions most especially 

 referred to by the author have been reduced to a suitable scale, and are given in Plate I. 



