136 SCIENTIFIC NOTES. 



as was anticipated. On this point, however, it is perhaps 

 premature to speak. Professor Newcomb, of America, holds the 

 opinion that I know some of my fellow-observers in the Colony will 

 be glad to hear, viz., that external contacts will give quite as good, 

 perhaps better results than internal contacts, an opinion which my 

 own observations and those of several of the New South Wales 

 observers fully bear out. 



As it is very important that all the photos, which are to be 

 combined should be measured with the same instrument, and under 

 similar conditions, Sir George Airy has constructed one specially 

 for the purpose. I had the pleasure of seeing it, and I will 

 endeavour in a few words to convey an idea of its form : — First 

 then, we have a strong rectangular casting of brass, about eighteen 

 inches long and seven wide. On this, near one end, is a plate-glass 

 scale seven inches long, and at the other end a convenient table 

 for holding the photograph. Projecting up from this casting are 

 six strong brass pillars, five inches long, and upon them a similar 

 but lighter brass casting, which is on the upper surface formed into 

 a bed, on which the microscope stage slides along. In this stage, 

 and fixed as to their relative distance, are two microscopes, one of 

 which is focused on to the glass scale and the other on to the photo. 

 The stage may be moved rapidly by hand, and has a screw for fine 

 motion. The measurement is made as follows : — Having placed in 

 the negative, its position is adjusted until the line in the microscope 

 becomes a tangent to the limb of the sun, to each limb of the planet, 

 and to the second limb of the sun, by simply sliding the microscope 

 stage along. When this is done, careful readings are taken of the 

 glass scale when the microscope line occupies each one of the 

 tangential positions. The difference between the extremes gives 

 the diameter of the picture, or sun ; and from the fom readings 

 between the two limbs of the planet and the two limbs of the sim, 

 the centres of the sun and the distance of the centre of the planet 

 from it are obtained. These measures will be repeated till a 

 satisfactory measure is obtained. It will be observed that the 

 Astronomer Royal in this arrangement makes no use of the reference 

 lines on the pictures, but trusts simply to the distance of centres. 

 One uncertainty — that of the perfect reliability of the position of 

 these reference lines — is done away with, but it will be observed 

 that, for this advantage, the gain to be derived from taking the 

 average edge of the whole sun instead of two points in it, which 

 may be enlarged or contracted in some photographs from accidental 

 causes, has to be given up. It is probable, however, that Dr. De 

 la Hue's micrometer will also be used. As I have no drawings of 

 this instrument, which is rather complicated, I may say that in 

 general plan it is similar to the American one, designed by Professor 

 Harkness, but instead of glass scales read by microscopes, it has 

 brass scales read by verniers. 



