LOCAL PAETICTJLARS OF THE TRANSIT OP VENUS, 1874. 79 



Eor the photograptic method, the following things in addition 

 to those mentioned are essential : — A small motion for adjust- 

 ment of focus must be provided, either in the mounting of the 

 object glass or camera box ; preferably in the former — for the 

 camera box must be so attached to the tube that motion round 

 its axis is impossible. In other words, the angle of position must 

 be absolutely fixed. Another provision must be made by which 

 every plate when under exposure will take exactly the same 

 position with reference to the object glass ; and the plan which 

 seems to me best adaj)ted to attain this end is to have the sup- 

 ports for the four corners of the plate attached to the camera 

 box and do away with the ordinary dark slide altogether. Of 

 course this involves having the end of the telescope in the dark 

 room, but this is easily done by making the side next the telescope 

 of dark light-proof cloth, through which the telescope may pass 

 and have the cloth tied round it. If properly done this will not 

 interfere with the clockwork, but will afford great facility of 

 manipulation, and the cloth hanging about the telescope will tend 

 to prevent vibration. 



Provision must also be made to have one fine wire, or better 

 two, at right angles to each other, stretched across close to the 

 sensitive plate ; in one or both provision must be made so that 

 the wire may be adjusted to the zero of position as in the micro- 

 meter. A piece of plate-glass, with a line and scale upon it, 

 may take the place of the wires, and then every picture would 

 not only have the zero line, but also a scale of reference printed 

 upon it, which would be a valuable check on distortion. The 

 care required in determining the true position of the wire or 

 glass may be gathered from the fact that if we take 10" as the 

 limit of error to be allowed, we shall require to know the 

 position of the end of the wire or line within (about) 1-lOOOOth 

 part of an inch. The methods of adjusting all the lenses will be 

 found in Mr. Eutherford's paper, and I need not add anything to 

 what he says. 



I have before alluded to the flashing shutter ; its details will 

 be best understood by reference to figure 5, in which A B C is 

 the diaphragm put in the telescope ; it has a hole in the centre, 

 d e, half an inch in diameter ; M M M M are pieces put on A B 

 C, as guides for the shutter n n n N. The shutter has a hole in 

 it betv^^een E, E, corresponding to d e, but half of it is covered by 

 the piece p which is fixed to the shutter, and the other half 

 nearly covered by the piece o o, which by means of the slots and 

 screws is adjustable, so that the slit E E may be varied at 

 pleasure ; but about l-50th of an inch is enough to separate p 

 and 0. S S S is a spring (common elastic) which causes the 

 shutter to flash past D E as soon as the string t is cut or burned. 

 If cut, it should be done with scissors, to prevent vibration in 



