254 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
1. With one diagonal slit, and between the two faces 
a. Canada balsam or linseed oil ; 
b. Air. 
2. With two diagonal slits intersecting 
a. In the middle of a basal plane (Ahrens) ; 
b. In the middle of the prism (Bertrand). 
III. Prisms which contain only the lamella of a doubly refracting 
medium. 
The requirements of an ideal prism are : — Plane polarized field, largest 
possible field of view, slightest possible refraction of the ray, smallest 
possible ratio of length and breadth, and least possible waste of 
material. 
In the following table the numbers 1 to 5 serve to estimate the value 
of the prism for the specific property indicated in each of the horizontal 
rows. The last column gives for each of these properties the most 
advantageous forms — viz. those marked with 4 or 5. 
Nicol Group. 
Air 
Prisms. 
Double 
Slit. 
ff 
£ 
Advantageous 
J4 
o 
c n 
Ph 
3 
*6 
p 
CD 
A § 
£ 
Form. 
'o 
1 
| 
o 
► 
o 
<D 
rO 
p 
ai 
o 
P 
o 
c3 
s 
f £ 
D 
a 
2 
H 
a 
< 
3 
<1 
ft 
s 
1. 
Plane polarized! 
field .. ../ 
3 
‘i 
6 
2 
2 
2 
2 
2 
3(1) 
2 
1 
Thompson, Hartnack. 
2. 
Field of view . . 
3 
3 
3 
2 
2 
1 
1 
4 
4 
1 
5 
r Plate prism, 
l Bertrand, Ahrens. 
3. 
Loss of light . . 
4 
5 
5 
5 
3 
2 
2 
3 
5 
2 
1 
fDove, Hartnack, 
{ Thompson, Ahrens. 
4. 
Displacement 1 
of ray . . . . / 
1 2 
5 
5 
5(1) 
5 
3 
1 
3 
5 
3 
4 
( Hartnack. Thompson, 
\ Ahrens (Dove). 
5. 
Ratio of length! 
1 
1 
1 
3 
3 
4 
4 
1 
3 
5 
2 
(Air-prism with double 
and breadth . . / 
\ slit, Gian, Foucault. 
6. 
Waste of mate- 
rial .. .. 
4(3) 
* 
1 
4 
4 
2 
4 
3 
3 
4 
5 
(Plate-prism, Dove, 
l Abbe, air-prism. 
Total.. .. 
17(16) 
' 22 
1 
20 
l7( + 4) 
19 
1 
14 
14 
17 
21 ( + 2) 
17 
1 
18 
At the author’s suggestion, Herr Halle, of Potsdam, has undertaken 
the preparation of the air-prism with double slit, referred to in the above 
table. It is not half so thick as broad and long, and would be useful 
as a polarizer. Another form suggested by the author may be described 
as a Bertrand prism with air slit. It is rather thicker than broad and 
long ; but as the field of view amounts to 15° it would be very service- 
able as an analyser. 
A new Camera Lucida.* — Herr G. Govi describes a new camera 
lucida, which consists of two rectangular equal-sided prisms of the 
same glass, one of which is smaller than the other. The hypothenuse 
face of the smaller, which is coated with thin gold-leaf, must be as 
large as the side face of the larger, on which it is fastened with Canada 
balsam or some other substance with refractive index as near as possible 
to that of the glass. 
The hypothenuse face of the larger prism is turned at 45° to the 
* Central-Ztg. f. Optik u. Mechanik, x. No. 22, p. 2G0. 
