112 Notices respecting New Books. 
noiselessly running combination, which may also be driven 
by means of a rubber ball pressed by hand, or by clockwork. 
The action is as follows:—For each revolution of the 
mirror 3 about the axis wa, the mirrors 2, 3, and 4 come into 
optical conjunction only once, hence the light from the seale 
can only once, during each revolution of 3, momentarily 
reach the eye a the observer. \, Now when the instrument 
mirror 4 is deflected through an angle a about the axis aa, 
the position of momentary ‘conjunction between the mirrors 
1 and 3.is shifted through an angle @ one way or the other, 
ies on the direction of deflexion relativ ely to the 
direction of rotation of the mirror 3; at the instant of this 
conjunction, however, the mirror 2, which rotates » times 
faster than the mirror 3, will be displaced forwards or back- 
wards through an angle na, so that the light which is reflected 
at 2,' 3, and 4 must come from a part of the scale whose 
angular distance is 2a one way or the other from the initial 
reading. 
The path-.of the ray from 2 to 3, 3 to 4, and 4 to 5, 
evidently lies in the same plane, and this plane follows the 
deflexion a of the instrument mirror 4 about the axis wa. 
Thus a mere increase in the velocity ratio n of the mirrors 
2 and 3 increases the sensitiveness of the arrangement, and 
does so without any increase of space or loss of light. 
Degerloch, Sibiamteils: ¢ ie shoals 29, 1904. 
XIII. Noitess respecting New Bais) 
Elektrische Fernphotographie und Ahnliches. Von Dr. ARTHUR 
Korn, a. 0. Professor an der Universitit Minchen. Mit 18 
Figuren im Text. Leipzig: 8. Hirzel. 1904. Pp. 66. 
I’ this pamphlet, the author gives an account of the extremely 
ingenious arrangements devised by him for obtaining synchronous 
rotation of two cylinders at stations a considerable distance apart, 
and of the transmitting and receiving instruments to be used in 
connection therewith for the telegraphic transmission of photo- 
eraphs, manuscripts, drawings, &c. The pamphlet is prefaced by 
an historical introduction, and concludes with a forecast of the 
probable improvements to be effected in the future. 
Théorie de Maawell et les Oscillations Hertzvennes.—La Télégraphie 
sans Fil. Par H. Porncaré. Paris: C. Naud. 1904 (Scientia 
Series, No. 23). Pp. vi+110. 
THe bulk of this volume has already appeared as No. 1 of the 
Scientia series, but here we have important additions in the shape of 
three chapters, one of which deals with coherers and magnetic 
detectors, another with the principles of wireless telegraphy, and 
a third with its practical applications.. The volume is a most 
readable and entirely non-mathematical account of the subject 
with which it deals. 
