VII. PHOTOGRAPHY. 423 



1853b. One of the Photographic Apparatus used in the 

 " Transit of Venus " Expedition. 



The French Commission for Observing the Transit of Venus 

 in 1874. 



1853c Photographic Revolver, used in. observing the 



transit of Venus. M. Janssen, Member of the Institute, Paris. 



1853d. Photographic Impressions, obtained with the 

 revolver. M. Janssen, Member of the Institute^ Paris. 



1854. Short Focus Mirror, spherical, for telescopes, cor- 

 rected by two lenses of homogenous media, for reflecting telescopes 

 and astro-photography. With pamphlet. 



Prof. Carl Wenzel Zenger, Prague. 



1856. Apparatus for the production of Photographs of the 

 Sun, after Dr. Oswald Lohse. A. Fuess, Berlin. 



1857. Stand with Equatorial Motion about a vertical and 

 horizontal axis for a photographic camera of 6" aperture, used by 

 the German expedition to Kerguelen's Island, for the observation of 

 the transit of Venus, 1874. (Photograph.) 



A. Repsold and Sons, Hamburg. 



The point of intersection of the horizontal and vertical axis produced 

 is at the same time the centre of movement of the equatorial system, which 

 consists of an hour axis and a declination arc. This arc is suspended from a 

 double arm fastened to one end of the horizontal axis and parallel to the tele- 

 scope, it turns round this arm remaining concentric with the centre of motion. 

 When claipped in the head of the hour axis, it turns with it, and compels the 

 double arm, and at the same time the telescope, to move equatorially about 

 the horizontal and vertical axes. By this arrangement the position of a thread 

 in the focus of the object-glass can always be controlled by the level attached 

 to the telescope. (See P. A. Hansen, " Beschreibung eines Fernrohrstatins," 

 &c., in the Berichten der Kgl. sachsisch. Ges. d. Wiss. Mathern. Phys. A., 

 1 Jul. 1870.) To enable the telescope to follow the diurnal movement there 

 is a screw moved by hand in time with the ticks of a clock. 



1858. Small Spectrograph. Simple apparatus for taking 

 the sun's spectrum, consisting of a camera (without objective), 

 and a Browning's pocket spectroscope. 



Prof. H. W. Vogel, Berlin. 



The small spectrograph serves for studying the chemical effect of the 

 different parts of the solar spectrum upon substances sensitive to light 

 and for ascertaining the varying intensities of the various parts of the soiar 

 light at different places and times. The slit is wedge-shaped, in order to 

 have more light at one side of the spectrum than at the other. The more 

 intense the chemical effect of a colour, the further it reaches towards the dark 

 end of the spectrum. The apparatus is held in the hand, and so directed 

 upon the sun that the spectroscope may throw no shade. No heliostat is 

 required. The exhibitor has been able to use the instrument on board ship, 

 whilst sailing from Brindisi to Ceylon. (Pogg. Ann., Bd. 156, p. 321.) 

 The exhibited apparatus has been made by Schmidt and Haensch, Berlin. 



