116 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
luminous impression, by neutralizing the rays emanating from the blue 
or violet parts of the object ; but the emulsion not being sensitive to 
the yellow, a general cutting off’ will result, both for the ground as well 
as for the object, and the image will not have gained. Retaining the 
same screen and substituting for the ordinary plate a plate sensitized 
for the yellow, the ground will be forcibly rendered, while the blues 
and the violets, partly reduced by the coloured screen, will act with less 
rapidity, and will be rendered on the plate in their proper value. In a 
word, the plate sensitized for the yellow will reproduce the object as if 
it were grey and black on a white ground. 
Where an object combines red and yellow colours it would be 
possible, at a push, to obtain a true rendering with a plate sensitized for 
yellow by commencing the exposure with a yellow screen, and continuing 
for the red with a screen equally red ; although, for the reasons already 
given, the substitution of one screen for the other would endanger the 
sharpness of the image. It would be better to sensitize for red and 
yellow and, according to the intensity of the former, expose with a deep 
yellow or orange screen. If blues and violets are found in the presence 
of yellows, oranges, or reds, it would suffice to use a plate sensitized for 
the least actinic colour (yellow or red), and as the plate is, of course, 
sensitive to the blues and violets, a yellow screen, pale or deep, could 
be used according as the more actinic parts of the object are more or 
less coloured. 
For development the author recommends hydroquinone with an 
alkaline carbonate and bromide, and the use of a feeble light in the dark 
room. 
(5) Microscopical Optics and Manipulation. 
Chromatic Aberration of Lenses.* — Prof. L. Weber gives the fol- 
lowing practical method of demonstrating in the simplest way from the 
law of refraction the chromatic aberration of a lens : — 
The aperture of a simple, moderately large lens L L (fig. 11) is 
covered with an opaque screen S S, in which two small holes o o , 1-2 mm. 
in diameter, are bored near the rim of the lens on the horizontal middle 
line. If a point of light be then brought into the focus F of the lens, the 
rays proceeding from the holes o o will be parallel. The eye at a dis- 
tance of 10 to 20 m. from the lens, looking along r , will then see one of the 
holes in the screen as a bright point of light, and in order to see the other 
hole will have to be displaced through a distance equal to the distance 
between the holes. If the distance be exactly equal to the distance be- 
tween the pupils of the two eyes, the two holes will be seen simulta- 
neously by both eyes. The smallest movement of the head will then 
cause the two points of light to appear unequally bright. 
The difficulty of getting a point of light, such as a needle-hole in a 
screen in front of a lamp, in the focus of the lens may be obviated by 
the use of a vertical slit about 1/2 mm. broad. 
The above observations will only be rigidly exact provided that 
monochromatic light (e. g. soda light) is used, and that the slit is exactly 
in the focus of the lens which corresponds to this particular colour. 
The observation will be quite different if the slit is illuminated 
* Central-Ztg. f. Optik u. Mechanik, xiv. (1893) pp. 241-2. 
