370 Prof. Maskelyne on the Mineral Constituents [Apr. 8, 



into two images ; the ordinary ray from one is superposed on the extra- 

 ordinary ray from the other, and the compound beam so produced is ex- 

 amined further. The means adopted to effect the desired object are, 

 however, very different, being much simpler in my method, whilst the re- 

 sults are superior. 



In Jamin's photometer the light which eventually reaches the eye is 

 comparatively feeble, and the field of view is very restricted ; the objects 

 themselves under comparison are seen direct through the instrument with- 

 out the interposition of a telescopic arrangement, and no means are taken 

 to prevent extraneous light from entering. The deficiency of light makes 

 observations by artificial light difficult, whilst when examining objects 

 illuminated by diffused or direct sunlight the eye is fatigued and bewildered 

 by the variations of shape, size, and colour assumed by the overlapping 

 objects seen through the instrument. In the photometer described in the 

 former part of this paper, there is abundance of light, and the observation 

 is made upon two luminous disks, which are magnified by means of a lens, 

 so as to appear close to the eye. It will be found much easier to detect 

 differences of colour between these two adjacent disks than to observe the 

 presence or absence of the coloured fringes in the central portion of the 

 field of Jamin's photometer. In the former case the eye has nothing to 

 observe but two uniform and purely coloured disks, changing from red- 

 green to green-red through an intermediate stage of neutrality ; in the 

 latter case the eye has to detect the stage of neutrality in the central por- 

 tion of the field, where the two images under comparison overlap, the at- 

 tention being distracted, and the sensitiveness of the eye weakened, by the 

 brilliantly coloured fringes which cross the adjacent objects. 



A direct comparison of the two instruments for sensitiveness shows that 

 the present photometer will detect much more minute differences of in- 

 tensity than Jamin's will, whilst it will work with tolerable accuracy in a 

 light too feeble to give any results with the latter instrument. 



April 8, 1869. 

 Lieut. -General SABINE, President, in the Chair. 



The following communications were read : — 



I. C( Preliminary Notice on the Mineral Constituents of the Breiten- 

 bach Meteorite." By Professor N. Story Maskelyne, M.A. 

 Communicated by Professor Warington W. Smyth, F.R.S. Re- 

 ceived March 2, 1869. 



This meteorite, which belongs to the rare class intermediate between 

 meteoric irons or siderites and meteoric stones or aerolites (a class to 



