Oct. 26, 1871] 
moderate care the reader will understand the points illus- 
trated nearly as well as if he saw the experiments them- 
selves. In great part Mr. Ball has devised these experi- 
ments himself, and thus in the well-worked field of 
elementary mechanics he has introduced much that is 
original in treatment, and in some parts—particularly in 
his lecture on friction—there will be found something 



































FIG. 3. 
more. On the whole the work is one that will amply 
repay perusal, both by teacher and student, and is a most 
valuable supplement to works on the theory of mechanics. 
Nor must we take leave of the volume without adding 
that its general appearance—due to paper, printing, and 
illustrations —is truly beautiful, and, in fact, we cannot 
callto mind any English book of the same class which 
will bear comparison with it in these respects. 
J. F. TWISDEN 

ON THE BEST FORM OF COMPOUND PRISM 
FOR THE SPECTRUM MICROSCOPE 
ie studying the spectra of coloured solutions and solid 
substances by means of the spectrum microscope, It 
is most important to employ prisms haying a suitable 
NATURE 

511 

amount of dispersion. It would be a very great mistake 
to suppose that the result is better with a very wide dis- 
persion. This, of course, makes the spectrum larger, but 
very greatly impairs the definition of the absorption-bands. 
Everyone who has had experience with an ordinary 
microscope must be well aware that a particular magni- 
fying power is best for each particular class of object or 
kind of structure, and that in some cases nearly all the 
important characters would be lost by employing too high 
a power ; but at the same time too low a power would be 
equally disadvantageous in other respects. This analogy 
holds good in the case of the dispersion of prisms. The 
power ought to be regulated by the character of the ab- 
sorption-bands. If they are dark, narrow, well-defined, 
and lie close together, as in the case of partially opaque 
crystalline blow-pipe beads of borax containing deposited 
crystals of oxide of lanthanum with oxide of didymium, 
a somewhat powerful dispersion is not only admissible, but 
quite necessary to separate some of the bands. If, how- 
ever, they are broad and faint like those seen in the spectra 
of many of the colouring matters found in animals and 
plants, a powerful dispersion spreads them over such a 
wide space, and makes the shading off so gradual, that the 
eye can scarcely appreciate the extra amount of absorp- 
tion ; whereas, when a lower dispersive power is used, a 
well-marked absorption-band can easily be seen. This is 
more especially the case with impure mixtures. I have 
found that when it was requisite to examine a mixed, 
somewhat turbid, coloured solution to detect, if possible, 
the presence of some substance which, when alone, 
gave a spectrum with distinct absorption-bands, no 
trace could be recognised by means of a prism of high 
dispersive power ; but it could be detected without any 
difficulty with a lower. In carrying on practical investiga- 
tions it is far more important to be able to succeed in such 
a case than to exhibit on a large and more imposing scale 
the spectra of a few substances which give dark and well- 
defined bands. There can be no doubt that it is a great 
advantage to have a number of prisms of different disper- 
sive power, so that in all cases the most suitable may be 
used; but at the same time some observers might not 
wish to have more than one, and thus it becomes impor- 
tant to decide what amount of dispersion is the best for 
the generality of cbjects—is sufficiently great to divide 
narrow, closely-placed bands, and yet not so great as to 
prevent our seeing broad and fairter. No magnifying 
power whatever is applied to the spectrum itself in the 
instrument now under consideration. 
As described in some of my former papers,* the com- 
pound, direct-vision prisms first made for me by Mr. 
Browning were composed of two rectangular prisms of 
not very dense flint glass, and three of crown glass, one 
being rectangular, and the others of an angle of about 
75°. This combination gives a dispersive power, which 
shows faint bands very well ; but is not enough to divide 
the narrow and close bands seen in the spectra of a few 
substances. Mr. Browning then made prisms of similar 
construction, only that very dense flint glass was employed; 
This combination gives about double the former disper- 
sion, which divides narrow and close bands admirably, 
but sometimes shows broad and fainter bands so very 
badly that they can scarcely be recognised. It thus ap- 
peared to me that, if only one compound prism be sup- 
plied with the instrument, the best dispersive power would 
be intermediate between these two extremes. At the same 
time much would depend on the particular purpose to 
which the instrument was applied, and also, to some extent, 
on the individual differences between different observers. 
Mr. Browning has described + the plan that he pro- 
poses for the measurement of the position of absorption- 
bands by means of a bright line, seen by reflection from 
the surface of the prism, moved backwards and forwards 
* Popular Science Review, vol. v., 1866, pp. 66—77; Brit. As. Report, 
1865 (pt. 2), p. 11. . a 
+ Monthly Microscopical Fournal, vol. iii. p. 68, 
