2 M. lludberg on the Refraction of the diffei-ently-coloured 



mediately by the mean of the verniers of the alidade 50" 

 centesimal, or nearly 16" sexagesimal. The instrument was 

 arranged in the following manner. The limb being placed 

 horizontally, the upper telescope attached to the alidade was 

 taken away and placed upon one of the arms of a lever, the 

 middle of which rested on the centre of the limb, and the other 

 arm of which was loaded by a counter weight equal to the 

 weiffht of the telescope. The whole was combined in such a 

 manner that in turning the alidade, we turned the telescope, 

 the object-glass of which described, in consequence, an arc of 

 a circle round the centre of the instrument. To this centre 

 was applied a rod of copper, forming a continuation of the 

 axis. This rod carried a plate about four inches in diame- 

 ter, above which there was fixed, by means of six screws, at 

 the distance of some lines, another plate, which could by this 

 means be made horizontal. In a hollow of this was a ring of 

 copper, which carrying a plate of ground glass, and having 

 its circumference toothed, turned by means of a screw, so that 

 the prism which was always placed on the plate of glass hav- 

 ing its edge in the centre, could be brought into such a posi- 

 tion that the refracted ray had its deviation a minimum. 



The sun's light was introduced into the dark chamber 

 through a small opening by means of a heliostate at the di- 

 stance of 33 feet from the centre of the repeating circle. The 

 opening in the window-shutter formed by two plates, one of 

 which was moveable by a screw, could be rendered more or 

 less narrow. 



The limb remained immoveable during the observations. 

 To be sure of this, the other telescope, which was on the lower 

 surface of the limb, had its cross wires directed to an object 

 which was situated on the other side of the Lake Malarn, at 

 a distance of more than 2500 feet. 



In order to measure the refracting angle of the prism, the 

 refracting edge being in the centre of the circle, the prism 

 was turned in such a manner that there could be seen succes- 

 sively, by means of the telescope, the two images of a mark 

 which was reflected from the two faces whose intersection 

 formed the edge of the prism. The mark was the bar of the 

 window of a house on the opposite bank of the Lake Malarn, 

 at the distance of more than 2500 feet. It is evident that by 

 turning the point of intersection of the crossed wires respec- 

 tively, upon each of the images of the mark reflected from the 

 two faces, tlie angle which the telescope describes is exactly 

 double that of the prism. 



With respect to the angle of deviation, the prism was turned 

 in such a manner that the anjile of deviation of the refracted 



