56 The Spectroscope : (January, 
prisms of diverse media, which, combined with a certain 
curvature of the faces of one or more of the prisms, might 
bring the intervals into a more or less perfect agreement 
with those of the normals. 
The true and most effectual remedy, however, appears to 
be the having recourse at once to the diffracted speCtrum in 
the construction of the spectroscope. There are, doubtless, 
great practical difficulties in the way of making a handy 
instrument on that principle; but it is believed that it will 
be found possible to surmount these more easily than to 
overcome those attending the obtaining of a refracted spec- 
trum which shall exactly correspond with the diffracted. 
It is comparatively easy to produce a diffracted spectrum, 
by means of a system of equidistant fine lines viewed 
through a telescope at a distance of about 12 feet. The 
difficulty is to secure a good large spectrum, from sucha 
system, within the compass of an easily portable instrument. 
Until that be accomplished, it would be well that every 
spectroscope formed with prisms should have its individual 
spectrum carefully compared with the normal spectrum 
formed by a standard instrument, and that a table should 
be constructed showing how much the positions of the 
principal fixed lines, and of the borders of the colours, 
differ in the refracted spectrum from what they are in the 
normal, such table to be attached to the instrument. 
It may not be amiss, however, to throw out a few hints 
as to the practicability of constructing a compact spectro- 
scope on the diffracting principle. Mr. Lewis Rutherford, 
of New York, has recently exhibited in London sets of 
equidistant lines ruled on glass, of which there are 1500 to 
the inch; and even with these fair diffracted speCtra can 
be obtained. But it is better to have the lines very much 
closer than these. It is not difficult to obtain copper wire 
1-200th of an inch in diameter. Suppose wire of this 
description to be so wound on a square frame as to leave 
between each strand an interval exactly equal to the 
diameter of the wire; this would give a hundred strands 
and a hundred equal intervals for each inch. A square 
frame, therefore, of 100 inches free space would contain 
‘10,000 strands of wire, and the like number of equal inter- 
vals. It is believed that such a frame might be constructed 
without much difficulty, and, were it once construéted, any 
number of photographs might be taken from it, reducing its 
size to one quarter of an inch. The photograph would thus 
have 10,000 equidistant lines in the space of a quarter of an 
inch. These photographs might be taken on thin plates of 
