32-1 Prof. Potter on the Interference of Light near a Caustic, 



for a small telescope, of length 4A inches, which magnified 

 rather more than twice, and adjusted to show distant objects 

 distinctly. It had micrometer lines at the common focus of the 

 object and eye-glasses crossed at an angle of about 35°, the 

 bisection of which was vertical. The image to be viewed in the 

 telescope was brought to the bisection of this angle, and the 

 measures were taken from the iris on one side of the perpendi- 

 cular ray to that on the opposite side of it ; the angle traversed 

 being divided by two, gave the distance from the perpendicular 

 ray for each iris. 



On the 17th of September, 1841, in two good experiments 

 with green light for the fixed line E, the extreme visible light was 

 found to extend to 42° 21|' in one experiment, and to 42° 21' in 

 the other, from the ray falling perpendicularly on the stream ; 

 and the calculated place of the primary rainbow, if it were at the 

 caustic, for this colour is 41° 40', therefore light was seen 41' 

 beyond the caustic. 



In two experiments with orange light, fixed line D, the extreme 

 visible light was found to extend to 42° 41' in one, and 42° 58|' 

 in the other from the normal ray. The calculation for the 

 caustic gives 42° 2' nearly, and hence the light was seen 39' and 

 56|' outside the caustic. 



Another experiment with yellow green light between the lines 

 D and E gave a distance 42° 33i', to which the light could be 

 traced. Measures of the first six maxima or bright bars were 

 taken, and though probably correct, yet on returning to the first 

 maximum for verification some derangement had occurred, so 

 that no argument can be taken from them to have weight. 



The stream was formed in passing through a small circular 

 aperture of g jth of an inch diameter, in a brass nozle on a pipe 

 furnished with a stopcock, which acted as a siphon from an 

 elevated pneumatic trough, so that the velocity of the stream 

 could be regulated by the stopcock. 



On October 2, I tried the effect of different velocities on the 

 position of the bars, and found with green light, when the inter- 

 ference bars were most distinct, the extreme visible light extended 

 to a reading on the circle of 269° 9', and when the velocity was 

 greater, it extended to a reading of 267° 35' ; and from the nature 

 of the reading, the light extended to 1° 34' further beyond the 

 caustic in the latter than in the former case. In another trial 

 with green light and the bars most distinct, the extreme visible 

 light gave a reading of 269° 4' ; but with a greater velocity and 

 the bars still visible, the reading was 267° 16', or the difference 

 was 1° 48'. When the full stream was allowed to run, the extreme 

 light extended still further, but the bars were not seen. 



These experiments, coupled with the measures of the natural 



