2 12 JOURNAL OF SCIENCE. 



extraordinary difference between the two without seeing it ; and I 

 am further able to show you the result of taking a star cluster with 

 an enlarging lens which makes the equivalent focus 564 in. or 47 ft. 

 The success of this new departure is very gratifying, because it shows 

 how much may be added to our knowledge of star clusters by this 

 method of direct enlargement. When possible it is much better to 

 enlarge in the camera at once than to enlarge the photograph after 

 it is taken, because there are always blemishes in the surface used 

 for the photograph which get enlarged with the picture. The first 

 photograph of Kappa Crucis did not cover a space of one-tenth of an 

 inch square, the star camera makes it 18 times larger and the 

 enlarging lens 324 times larger. Where extreme accuracy for 

 measurement is required, as in these cases, the gain is even greater 

 than these numbers indicate, and under the microscope the magnified 

 image may be again magnified 50 times. The smaller one will 

 bear no greater power because it is the imperfections in the surface 

 and image that limit the magnifying power that can be used, the 

 faults of the photo, surface being relatively so much less important in 

 the enlarged picture than in the small one, and this photo., with the 

 enlarging lens, speaks volumes for the stability and accurate motion of 

 the telescope which on such a large scale gives perfectly sharp star 

 discs. The clearness of these star discs affords also a good test of the 

 effect of colour, and there are many coloured stars in it to indicate 

 what T mean ; it will suffice to indicate two stars — a red and a blue ; 

 the red star 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 

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

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

 the eye and the photograph, makes a difference of 3 magnitudes. 

 I think the members are aware that the photographs T exhibited here 

 last year were made with a 6 in. Dalmeyer portrait lens, and my 

 object now is to bring before you the state of preparedness of the star 

 camera for the work of charting the heavens, and some examples of 

 the actual work, at least plates taken of the dimensions and conditions 

 of the actual plates, 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. The one I have was courteously sent to 

 me by Admiral Mouchez, the Director of the Paris Observatory, 

 untested, as a sample : the process of testing those to be used being a 

 tedious one, and it will therefore be still some time before the 

 approved ones are available, but for our present purpose it 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 fines at 

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

 silver through ; these lines are about yu of an inch apart, equal to 5 

 minutes of an arc. Each line is numbered. This reseau is used in 

 this way : it is placed face upward in a box the exact counterpart of 

 the plate-holder in the telescope, upon this is then placed a sensitive 

 plate and the box is then closed and put in front of the object glass 

 of the camera. A small electric lamp of 2h candles is then placed in 



