62 H. C. RUSSELL. 



the telescope, which under such a magnifying power gives perfectly 

 sharp star discs. The clearness of these star discs, many of them 

 representing coloured stars, enables us to see distinctly the effect 

 of different colours on the size of the star discs. There are two 

 conspicuous stars, a red, and a blue, the red star to the eye is 

 fully a magnitude brighter than the blue, Herschel called it 

 9th magnitude, and the blue one 10th magnitude, the red one 

 in the photograph appears of the 11th magnitude or two magni- 

 tudes less, and the blue one appears of 9th or one magnitude 

 greater, in other words, the difference in colour as estimated 

 by the eye and the photograph, makes a difference of three mag- 

 nitudes in the stars. 



As I have just stated the photographs exhibited here last year 

 were made with a six inch Dalmeyer Portrait-lens. My object 

 now is to bring before you the state of preparedness of the Star 

 Camera for the work of charting the heavens, as well as some 

 examples of the actual work, plates taken of the dimensions and 

 under the conditions of the plates which will be used for the chart, 

 and only differing from them in that the reseau or grating of 

 lines, though ruled and made by the same machine as those that 

 are to be used, has not been tested in Europe, as all must be 

 before they are accepted. These are in fact experimental plates. 



The reseau I have, was courteously sent to me by Admiral 

 Mouchez, the Director of the Paris Observatory — as an untested 

 sample. The process of testing those to be used being a tedious 

 one, it will take some time before the approved ones are available, 

 but for our present purpose the one I have answers admirably. 

 It consists of a piece of plate glass with a thick coating of silver 

 from solution on one side, on this silver, two sets of lines at right 

 angles have been ruled with a sharp point, which has cut the 

 silver through, the lines are about two-tenths of an inch apart, 

 which is equal to five minutes of arc, and each line is numbered. 



The re'seau is placed face upward in a box, the exact counter- 

 part of the plate holder in the telescope, upon it is then placed 

 a, sensitive plate, and the box is closed and put in front of the 



