66 J. Watcrhouse — Photography in connection with- [No. 2, 



diately drew down the shutter and allowed a momentary exposure of the 

 sensitive plate to the solar rays during the passage of a slit in the shutter, 

 the width of which could he increased or diminished at will from nil to "5 

 of an inch hy means of another slide worked by a screw connected with a 

 graduated scale. The rapidity of motion of the shutter could also be regu- 

 lated by increasing or diminishing the tension of the spring by means of 

 a screw. 



"When the shutter was down the solar rays were quite cut off ; but by 

 a simple arrangement a circular aperture above the exposing slot could be 

 brought into a position concentric with the axis of the telescope, thus per- 

 mitting the whole bundle of rays to pass uninterruptedly through the 

 camera and enabling the image to be examined for focussing, &c. 



The camera of the photoheliograph was constructed to take plates six 

 inches square. The position of the image on the plates was regulated by 

 means of a finder fixed on the outside of the telescope tube and consisting 

 of a lens throwing an image of the sun upon a screen made of talc covered 

 with paper, and adjusted so that when the enlarged image was in its pro- 

 per position on the ground glass of the camera the finder image just filled 

 a square ruled on the talc screen.* 



Janssen Slide. — A repeating arrangement for taking several pictures on 

 one plate, designed by Dr. Warren De la Rue on the principle proposed by the 

 eminent French astronomer M. Janssen, and known as the Janssen slide, also 

 formed part of the equipment. This arrangement having been fully described 

 and figured by Dr. De la Rue,f it will suffice to say that it consists of a circular 

 wooden case about 12 in. in diameter and 2 in. deep, with a removable shutter 

 in front and constructed so as to be fitted on to the camera in the position 

 occupied by an ordinary dark slide. Revolving on a central axis within this 

 case is a metal disc or plate-holder, with 60 radial slots and as many circular 

 spaces racked in its edge, carrying the sensitive plate held between rings 

 strongly electroplated with silver. Outside the case, in front, a second smaller 

 disc revolves just outside the shutter and is provided with a radial opening 

 capable of being opened or closed at pleasure, so as to regulate the exposure 

 by admitting more or less light to the plate through a radial slit cut in the 

 shutter of the slide, about 1 in. long and exactly corresponding in position 

 and width to the sixtieth part of the circumference of the plate. The axis of 

 this exposing disc passes through the case and carries a pin which fits into 

 the slots in the edge of the revolving plate-holder and is turned, from outside 

 the case, by means of a winch arranged with gearing, so that it may be 



* The screen originally supplied with, the instrument was of parchment, but as 

 this was found to expand and contract with the variations of moisture in the air it 

 was advantageously replaced hy the talc and paper screen. 

 t Roy. Ast. Soc. Monthly Notices, May 1874. 



