Se eee 
TRANSACTIONS OF THE SECTIONS, 27 
brought into the line of vision singly or in combination, and of being rotated sepa- 
rately or as a whole. On the stage of the instrument a quartz plate is placed with 
a diaphragm, giving the original beam of polarized light. By means of the first 
double-image prism this beam is divided into two, showing the component parts of 
which the tint is composed. A second prism can be now applied ; this will divide 
each beam into two parts, making four images, and the third double-image prism 
will give eight images ; so that the original beam can be split up into eight com- 
ponent parts. The two quartz plates can then be introduced between the prisms, 
giving additional power in the analysis of colowrs. 
This instrument also has a small arm for carrying a pocket-specttoscope, which 
can be brought into the line of vision over the ordinary analyzer to enable the 
operator to analyze the colours by means of the spectroscope. 
On a New and Simple Form of adjustable Slit for the Spectroscope. 
By 8. C. Trsrey. 
The end of the spectroscope-tube is screwed on the outside so that the cap on 
pee turned round is gradually drawn down; this being bevelled on the inside, the 
bevel acts upon the ends of the jaws of the slit, which are correspondingly bevelled, 
and so closes the slit, a spring wire being used to open the slit when the cap is 
unscrewed, 
Advantage is taken of the cap, and a glass cover is inserted so as to keep the slit 
free from dust, the usual pull-off cap or case being entirely dispensed with ; every 
part of the instrument is easily accessible, and at the sanie time is solid and compact. 
By this means a really practical pocket-spectroscope can be constructed, and no 
loose cap or cover is required to protect it when out of use; it is always ready at 
a moment’s notice, and the utmost precision is obtained in the adjustment of the 
slit without any very delicate screws being employed. 
ELEcTRICITY AND MAGNETISM. 
On some Peculiarities in the Electric Discharge from a Leyden Jar. 
By Dr. W. Frvprrsun, Leipzig. 
The author showed a number of photographs obtained by projecting an image of 
the discharge-spark of a Leyden battaty on to a sensitive plate by means of & 
rotating concave mirror, and illustrated by reference to them the nature of the 
movement of electricity which must be conceived of as occurring when such 
discharges take place under varying circumstances. 
When the Leyden jar is discharged through a circuit of small resistance, the 
way in which the discharge takes place is that the opposite electricities of the two 
coatings do not neutralize each other at once, but as it were pass through each other 
several times in succession in a continually smaller and smaller quantity, so that at 
definite successive instants the jar appears to be charged wee positively and. 
negatively, hut each time less and less strongly. This is the oscillatory discharge, 
which reveals itself in the photographs by maxima of illumination following each 
other at equal distances. The known rate of rotation of the mirror gives the 
duration of the oscillation. Experiments show that this duration is proportional to 
the square root of the number of jars* (capacity of the battery) employed, that it 
depends on the form of the discharging circuit, and increases with the length of the 
circuit and decreases with its thickness in a complex ratiot, but is nevertheless 
independent of the resistance of the conductor. On the other hand, however, the 
number of oscillations seems to be essentially affected by this resistance ; when the 
resistance increases, the number becomes smaller, and, with a certain very high 
resistance (which the author calls the “limiting resistance”), the oscillatory 
character of the discharge ceases{. When this resistance has been reached, we 
* Poggendorff’s ‘Annalen der Physik u. Chem.’ exvi. 153. t Ibid. exvi. 164-169, 
{ Represented in reverse order in Pogg. ‘Annalen,’ exyi. pl. i. figs. 25-28. 
