12 REPORT — 1842. 



ll9ths.j|; ro . ducts - 

 I. Series. 



Indices of 17ths. 8. 12. 1. 10. 15. 14. 4. 6. 9. 5. 16. 7. 2. 3. 13. 11. 



In a similar manner to the last example, the circulate for any prime denominator 



whatever may be found. 



Extracts from a Letter on the state of the Observatory at Glasgow (25th 

 June, 1842). By Professor Nichol. 



" The members of the Physical Section will be pleased to know, that our great transit 

 circle, made by Ertel, is now in operation. The object-glass of the telescope is 6 - 2 

 inches, and the diameter of the directed circle 3 J feet. The construction of this circle 

 is somewhat different from those formerly furnished by the same celebrated artist. 

 Its circles are placed outside of the pillars on which it rests, and the alidada, instead 

 of verniers, carries microscopes. The position of the alidada, secured roughly by a 

 clamp, is ascertained on any observation, by the state of a delicate level, each of 

 whose divisions values 2*"4 of space, so that the power of the eye to subdivide this 

 quantity along the breadth of a division of this level gives the ultimate limit of the 

 precision of the instrument for a single observation. Two fixed collimators, watched 

 by levels, each of the divisions of which values 3", enable the observer to determine 

 the position either of the horizontal line or of a line, at a known angle with the hori- 

 zontal line ; and it will be clear that observations on both collimators necessarily indi- 

 cate the horizontal collimation ervors of the middle levers of the telescope, as well as 

 the errors arising from flexure. 



" Our meteorological department is nearly complete ; and I have just received no- 

 tice of the arrival of the declination- and horizontal-force magnetometers for our mag- 

 netic pavilion, from Meyerstein of Gottingen." 



On the Matliematical Expressions which lead to an explanation of all the 

 ordinary Phenomena in Optics. By Professor MacCuleagh. 



On a Neio Property of the Bays of the Spectrum, with Observations on the 

 Explanation of it given by the Astronomer Royal, on the Principles of the 

 Undulatory Theory. By Sir David Brewster. 



If we eover half the pupil of the eye with a thin plate of any transparent body, and 

 thus view a prismatic spectrum, so that the rays which pass by the plate interfere with 

 those which pass through it, the spectrum is seen crossed with beautiful black and nearly 

 equidistant bands, whose breadth, generally speaking, increased with the thinness of 

 the plate. If the edge dividing the ray were directed to the red end of the spectrum, 

 then fringes were seen ; but no such fringes appeared when it was turned to the 

 violet end of the spectrum. One peculiarity of these fringes, not before noticed, was 

 that they had not the forms of bands, but rather the appearance of screws, or dotted 

 black lines, or as if they were formed by the shadow of a plate of metal perforated by 

 small openings. This, which appeared to be a new property of light, and to indicate 

 a polarity in the single rays of light, when separated from each other by refraction, 

 he had commented on at the meetings of the Association at Liverpool and Bristol ; 

 and Mr. Airy, the Astronomer Royal, had given a paper and two publications on the 

 subject, in which he endeavoured to account for this upon the undulatory theory, 

 arguing that the appearance and magnitude of the fringe depended upon the diameter 

 of the pupil, or of the object-glass. Sir D. Brewster said he had repeated all his ex- 

 periments under every variety of form, varying the diameter of the pupil from its 

 greatest expansion to its greatest contraction, and the diameter of the object-glass 

 from four inches to a quarter of an inch, and the fringe remained utterly unaffected 

 by these variations. He further found, that these fringes varied in magnitude with 

 the distance of the eye from the refracting body, and not with the magnitude of the 

 pupil. He stated several other results, all of which, he thought, could not be explained 

 on the principles of the undulatory theory. 



