1 38 Royal Astronomical Society. 



siderable angles of incidence the order of magnitude is reversed. 

 The difference is far more conspicuous in the former case than in the 

 latter, and in that case consists principally in the greater extent of 

 the ragged edge. When the incident light is polarized at an azimuth 

 of 45°, or thereabouts, and the transmitted light is analysed so as to 

 extinguish the light transmitted near the point of contact, there is 

 seen a central dark patch surrounded by a luminous ring. 



ROYAL ASTRONOMICAL SOCIETY. 



[Continued from p. 72.] 



Nov. 10, 1848. — The Method in Use at the Cambridge Observa- 

 tory of Measuring Diiferences of Right Ascension and North Polar 

 Distance by an Equatoreal provided w^ith Clock-movement, and of 

 Correcting the Observations for Refraction. By Professor Challis. 



" Diiferences of north polar distance are usually measured by the 

 Northumberland equatoreal, by means of a small sector of a large 

 circle, on the limb of which are inscribed equidistant divisions, se- 

 parated by an arbitrary but ascertained interval. A similar sector 

 can be clamped to any part of the hour-circle, and differences of 

 right ascension measured in an analogous manner. This is effected 

 by an arrangement contrived by Mr. Airy (who contemplated the 

 kind of observation here described), by which the instrument may 

 be moved about its polar axis independently of the hour-circle, while 

 the latter is carried nearly at the rate of sidereal time by a clock. 

 The hour-circle sector has been substituted for the hour-circle itself, 

 because the divisions of the latter are on brass, and not so well- 

 adapted for accurate bisection as those of the sector, which are on 

 white metal ; and because the equidistance of the divisions, which is 

 the essential condition, is more likely to be secured in a small por- 

 tion of a circle than in a complete circle. The intervals of both sec- 

 tors are subdivided by microscope-micrometers. The following is 

 the method of taking the observations. 



" It is generally required, and always desirable, to measure simul- 

 taneously differences of right ascension and north polar distance. 

 Accordingly the object is bisected by the equatoreally adjusted wire, 

 very near the transverse wire, so that the rate of the clock, gaining 

 or losing as the case may be, soon brings it upon the latter wire, the 

 observer taking care in the meantime that it remains bisected by the 

 other. The instant of simultaneous bisection by the two wires is noted, 

 and the microscope-micrometers of the two sectors are then read off 

 in integral intervals and revolutions, and parts of a revolution. This 

 process is commenced with the star, or point of reference ; the object 

 referred is next observed in the same manner, and so on alternately, 

 the series concluding with the reference star. In case the compared 

 object be too faint for observation with micrometer wires, the prac- 

 tice with the Cambridge equatoreal is to use a diaphragm bounded 

 by straight edges at right angles to each other, and the object being 

 placed near the angular point in the prolongation of the edge which 

 is equatoreally adjusted, the instant at which its centre is brought 



