88 LOCAL PAETICULABS OF THE TRAKSIT OF VENUS, 1874. 



"As to the size of the negative, we may assume that when in the micro- 

 scope it should seem the same size as the sun seen through a telescope witli a 

 magnifying power of 240. To effect this with a microscope magnifying five 

 times, the diameter of the negative must be that formed in the image of a 

 telescope 40 feet long, or 4| inches ; but if the negatives can certainly be made 

 to bear a higher power a smaller image may be advantageous, and if it will 

 bear a power of 10 on the microscope it might be 2 inches (I have seen a 

 negative that would bear a power of 50). In order to take the photographs 

 of the size mentioned we must either have the telescopes 40 feet long, as 

 proposed by Professor Winlock, or magnify the image with an enlarging 

 combination. But the 40-feet telescope will be too long to point direct at the 

 sun, and Professor Winlock suggested it should be fixed horizontally north 

 and south, and the sun reflected into it by a moving mirror in front of the 

 object glass. It seems to ofier decided advantages over the other method. 



" 1. It gives an image free from all distortion, except such as can be accu- 

 rately determined and allowed for. 



"2. Grranting the plane surface of the reflector preserved, it affoi'ds the 

 most certain means of determining the angular value of any linear space on 

 the picture. 



" 3. It aifords the means of determining the angle of position with great 

 certainty. 



" 4. The plate-holder may be firmly fixed on a stone pier in the dark room. 



In reference to the methods of carrying out these plans, he says, " the 

 reflector of the heliostat may be made of glass, silvered glass, or speculum 

 metal, the requirements being that it shall not change its shape, or become 

 curved by necessary exposure to the sun. To reduce the probability of this, it 

 is best to use a metallic reflector when the exposure being only for about 

 one second in every minute (if photos are taken at the rate of one per minute) 

 or l-60th of the whole time, the mirror will not change much, and may in 

 the meantime be carefully covered, or, if used for focusing, covered with blue 

 or green glass. The heliostat should be worked by clock-work, free from 

 all vibration produced by the wheels striking together. It is probable the 

 reflector may at all stations be placed within one foot of the object glass. 



" The object glass should be five or six inches diameter, forty feet focal 

 length, and corrected not for visual but photographic rays ; it must not be 

 supported by the tube, but by a stone or brick pier the same as the plate- 

 holder. The tube is only useful to cut oS extraneous light. The object glass 

 should have screw motions to adjust it to the meridian and focal distance. 



" For southern stations the heliostat and object glass should be south of the 

 plate-holder. 



" Zero Lines. — The pier which supports the plate-holder must be so formed 

 that a plumb line may be suspended close to the sensitive plate and passing 

 through the middle of focal point. The plumb line must be a very fine 

 platinum wire, with a heavy bob. Provision must be made in the support 

 for turning the wire round so that any want of perfect flexibility may be over- 

 come by reversing the wire at each picture. The perfect verticality of this 

 wire must be carefully investigated and secured. A horizontal wire must also 

 be placed crossing the vertical one (without touching it) in the centre of the 

 focal point. Exposing the plate involves some mechanical difilculties. Equal 

 exposures of every part of the picture may be attained by cutting oflT the sun- 

 light in front of the objective by a narrow slit moving past it, but in this 

 arrangement the image will be elongated by diflVaction in a direction at right 

 angles to the slit. To overcome this difficulty it appears desirable to put the 

 cut-off or exposing arrangement within one or two feet of the face ; but if 

 this is operated in the usual way, viz., by having a narrow slit moving across 

 the rays by the action of a spring, the last j)art is exposed less than the first 



