ASTRONOMICAL PHOTOGRAPHY. 171 



optical instrument which you will see in the lower gallery called a 

 siderostat or heliostat, which was adjusted to reflect the image of the 

 sun constantly in the axial direction of the telescope. This has some 

 advantages and some disadvantages the advantage is that distortion 

 is reduced to a minimum. Our Chairman reminds me that I ought 

 not to forget to call attention to a set of instruments for the optical 

 and photographic observation of the transit of Venus, kindly con- 

 tributed by the Astronomer Royal, which will be found on the 

 balcony. It is a complete equipment, and it is well worth inspecting 

 because it shows how very much had to be done for eveiy fully 

 equipped station in order to secure accurate results. In the Kew 

 photo-heliograph the diameter of the sun on the occasion of the late 

 transit of Venus would be nearly four inches ; the diameter of Venus 

 is i '26 of an inch, and the maximum displacement of Venus nearly 

 one-tenth of an inch. Measurements can be made accurately within 

 o'2 of a second of arc, by means of the micrometer which I have 

 described, but an error to that extent would only produce an error of 

 less than 0^04 second of arc in the deduced solar parallax. It is to be 

 hoped, thereiore, with what has been secured in these observations 

 that we shall obtain the solar parallax in other words the earth's 

 distance from the sun to a very great degree of accuracy ; more- 

 over we have another eclipse coming off in 1882, and the same 

 instruments or similar ones will be ample to obtain the observations 

 requisite for a still closer approximation to the true parallax. 



I now wish to call your attention to some photographs of the sun 

 on a scale of four feet for the sun's diameter. They were obtained 

 with the Cranford instrument, which is only thirteen inches in diameter, 

 the image passing through a secondary magnifier, by which it was 

 magnified, before it fell on the sensitive plate. Not only is it possible 

 to photograph the sun directly on this large scale, but it is also 

 possible, as you will see by this print, made in 1862, to obtain solely 

 by the action of light and electro-metallurgic processes, printing 

 blocks without the touch of a graver which will print the sun spots. 

 This appeared in the monthly notes of the Astronomical Society in 

 1862, but unfortunately it was very badly printed. 



You are all aware that many attempts have been made to connect 

 solar phenomena with meteorological phenomena, and there can be no 



