1872.] Light. | 397 
we place, first, only so much of the condenser as will bring to parallelism the 
rays diverging from the source of light, they will then pass on to the remaining 
lens after reflection, exactly as they would have done had the condenser 
occupied its usual position, and will evenly illuminate its surface in conta& 
with which the ‘ object’ finds itself, and thus they will produce a field evenly 
illuminated. It is this set of conditions which has defeated the attempt 
frequently made to use large condensers with small pictures, by moving the 
latter forward on the cone of rays coming from the former. For use with the 
vertical lantern I find the following combination of lenses makes a very good 
condenser :—Nearest to the light a plano-convex lens of 43 inches diameter 
and focus of g inches; next to this, with its flat side also towards the light, a 
plano-convex lens of 5 inches diameter and 7 inches focus; then beyond the 
mirror a plano-convex lens 5 inches diameter and 8 inches focus, with its flat 
face upwards or away from the light. One might naturally suppose that 
with so rude a lens as a watch-glass full of water a very ill-defined and un- 
satisfactory image would be formed; but this is not the case, because the rays 
of light which pass through any point of the object do not spread over the 
whole of the obje&-glass (as is the case with the rays from an object which 
enter an ordinary portrait camera or telescope), but fall upon a small element 
of the lens only. They therefore acquire only the spherical and chromatic 
error proper to such a small element, and not that due to the entire lens. Any 
one familiar with the action of lenses for other purposes will be astonished by 
the accuracy of the image obtained in the lantern with a common plano-convex 
lens whose curved face is turned towards the object.” “It is, of course, the 
previous concentration and focalising of the rays by the condenser which 
accomplishes this good result in the manner named.” 
The ‘Journal of the Franklin Institute”? reports a lecture by Professor 
A. M. Mayer, at the Sheffield Scientific School, which included a very 
beautiful acoustic illustration of the method of determining stellar motions by 
the spectroscope. The lecturer dwelt upon the analogy between sound waves 
and those of light. Whenever one of the coloured waves composing white 
light is lengthened, its colour is modified in the direction of the red end of 
the spectrum, and vice versa. The motions of a star when at right angles to 
the view is susceptible of telescopic measurement ; but, the lecturer continued, 
the motion of a star when in the direction of the line of sight can only be 
measured on the foregoing principle, the standard being the displacement 
from their normal position of certain lines in itsspecrum. This principle is 
capable of illustration by means of waves of sound. With the lantern an 
image of a tuning-fork beating 256 times a second is thrown upon the screen. 
By the side of one of the prongs, and just touching it, a cork-ball is suspended 
by a filament of silk. On sounding a second fork tuned in accurate unison 
with the first, anywhere in the room, even at a distance of 30 feet, the first is 
thrown into vibration, and the image of the cork-ball projected a foot or two 
away from the prong. When, however, the second fork is carried rapidly to or 
from the first, no motion of the cork is perceived, the wave of sound being 
shortened or lengthened by an amount sufficient to throw it out of unison with 
those produced by the first fork. If a fork vibrating 254 times a second 
be now rapidly approached to the first, the waves are so much shortened as 
to be in unison with it and to throw the ball into motion. A fork vibrating 
258 times a second carried rapidly from the first, exhibits by the lengthening 
of the waves the same phenomenon. 
Microscopy.—Dr. James Murie, F.L.S., &c., continues his paper* on the 
classification and arrangement of microscopic objects in a communication to the 
Royal Microscopical Society. The principles of classification are dwelt upon 
at some length, a matter of especial value in the case of large and varied 
collections. Several instances are given of the mode of settling the proper 
place of objects which might be entitled to be located in any one of several 
departments. For example, of egg-shell the author says, ‘“‘ Although com- 
posed of the salts of lime, it is virtually an animal produ@, and therefore to 
* Monthly Micro. Journ., vol. i., p. 69; vol. vii., p. 201. 
