A Neiv Illuminating Apparatus. By E. M. Nelson. 445 
we have in view. On the same piece of wood that carries the slit 
another piece of wood slides which carries the prism. This piece of 
wood rotates on its axis ; connected to this is another piece of wood 
which rotates round the prism as a centre. This holds a photo- 
graphic lens known as a Wray 5x4 R.R., working at //5 * 6. Lastly 
there is a card on a movable stand. The method of use is as follows. 
A strong beam of light is condensed on to the slit S, fig. 52, 
which is kept about 1/12 in. open, by means of one of my new bull’s- 
eyes, B to which an additional lens has been added. From the slit the 
light passes to a dense flint prism P which by means of its rotating 
holder is set at minimum deviation. 
The Wray photographic lens R, which is eminently suitable for 
this purpose, both on account of its being corrected for rays high up 
the spectrum and also on account of its large aperture, is rotated 
Fig. 52. 
P 
round the prism until the refracted beam falls directly on it. The 
prism and this lens, which is attached to the same fitting, are now 
both moved to a distance from the slit equal to twice the focal length 
of the lens, the image of the slit, i. e. the spectrum, being focused on 
the card D, which is placed at a similar distance from the lens, and on 
the other side of it. The card is then moved so that the kind of 
light required may pass through an aperture in it to M the mirror 
of the Microscope. The apparatus has also been made in metal (see 
fig. 5), attached to a firm bull’s-eye stand ; another form is also made 
in wood, suitable for direct illumination without a mirror. The con- 
densing system is not absolutely necessary ; by placing the edge of 
the lamp-flame close to the slit a good light can be obtained, but the 
light is more intense when the condenser is used. We then proceed 
with the manipulation of the Microscope in the usual way, the slit 
