8o 
Mr. §. F. W. Herschel on the action of 
zero, 2 a being the constant distance between the poles. In 
order, however, to put this to a satisfactory examination, 
accurate measures must betaken, which, in the case of nitre, 
from the minuteness of the system of rings, presented at first 
some difficulties. These I obviated, after many fruitless 
trials, by a mode of observation which I have found extremely 
convenient and accurate, and which applies particularly well 
to the present purpose. It consists in projecting the rings by 
solar light on a screen in a darkened room, by which means 
they may be magnified to any required extent, examined] at 
perfect leisure, and in all their phases, and measured or 
traced with a pencil with the utmost exactness and facility. 
They may be thus exhibited too to a number of spectators at 
once; a thing which may prove serviceable to the Lecturer, 
for which reason I have subjoined to this paper a brief descrip- 
tion of the apparatus I employ. 
Having cut a very perfect crystal of nitre at right angles 
to its axis of crystallization, and adjusted it properly on this 
apparatus, the rings were projected on a large sheet of paper, 
stretched, while moist, on a drawing board, by which means 
it assumes a truly plane surface by the contraction it under- 
goes while drying. The poles were then marked, and the 
loci of the successive maxima for red rays carefully out- 
lined. The screen being then removed, a series of lem- 
niscates were laid down by points, having the same poles 
and one common point in each, chosen where the tint was 
most decided. It is unnecessary to give any comparative 
statement of measures in the observed and constructed curves, 
as the points, graphically laid down, uniformly fell on the 
pencilled outlines, or, in the few instances to the contrary. 
