52 Proceedings of the Royal Society of Victoria. 



sky with two sets of plates, will require nearly 42,000 

 plates ; the Melbourne zones will require 2298. 



It has been found that the gelatine tilms of the plates 

 sometimes shrink unequally in drying after development. 

 Such a thing would of couise vitiate the subsequent 

 measurements of the stars' positions as shown on the plates. 

 To obviate this, the following plan has been adopted : —A 

 glass plate exactly the same size as the photographic plate, 

 ii^ X 6^, is silvered on one side. This silvered siuface is 

 ruled into squares by extremely tine lines, five millimetres 

 apart; the lines show as clear glass, and allow light to pass 

 through. Each photographic j^late before being used is 

 placed in contact with this ruled plate and exposed foi a 

 second or two to parallel rays of light, which, passing 

 tlirough the rulings, impress a latent image on the film, and 

 when developed after exposure to the stars in the telescope, 

 exhibits the stars on a j)late traversed by a network of 

 extremely fine lines. 11, now these lines are exactly five 

 millimetres apart after the film has dried, it shows that 

 there has been no distortion in shrinkage ; if not, the 

 amount of shrinkage can be measured. The silvered and 

 ruled })late is called " the reseau," and every plate, before 

 being used for chai'ting, has to be exposed to the reseau as 

 described. 



Having explained the general scheme, I propose now to 

 give a brief description of the arrangements made at our 

 Observatory for the Melbourne ])ortion of the work. The 

 congress left it to the several asti-onomers to get their telescopes 

 constructed on any plan and by any maker they chose, 

 stipulating only that the jjliotograph telescopes should all 

 be of the same optical character and dimensions, viz., thirty- 

 three centimetres aperture, and thirteen feet focal length, 

 and the object glass to be corrected for the wave length 

 about G. Several instruments were made in Paris, some in 

 Germany, and some in America. Those for British and 

 Australian observatories weie made b}' Sir Howard Grubb, 

 of Dublin, and ours is one of the.se. It consists of a twin 

 telescope on a massive equatorial stand of the German form, 

 with an unusually long declination axis to allow of plenty 

 of room about the eye end when the telesco])e is ])ointed to 

 the meiidiau Both the photograph and guider telesco])es 

 are made with strong steel tubes connected one with another 

 in a most rigid manner. The ])hotogra])hic telescope is 

 ])rovid(Ml with a metallic plate holder, having all necessary 



