i;o SECTIONPHYSICS. 



the sun exactly coincides with the centre of the divided circle whose 

 axis is a hollow cylinder, and the north point made to correspond 

 with the zero of the circle. By drawing out a slide we can bring 

 the periphery of the sun under the cross wires of the microscope, and 

 by reading off on a vernier can see the distance'passed over, and obtain 

 the value of the radius of the sun in inches and decimal divisions. 

 The photograph is then crossed over to the other side so as to eliminate 

 any error of centring, and then we have the measure of the whole 

 diameter on the arbitrary scale. We then bring any sun spot under- 

 neath the microscope by turning the circle and drawing out the slide, 

 and we read on the circle its angle of position and on the slide its 

 distance from the centre. With these measurements after they are 

 reduced we are enabled to ascertain the helioscopic position in 

 latitude and longitude. But there is another disturbing cause to the 

 application of photography to exact measurements in astronomy. 

 There is always some optical distortion. In order to ascertain its 

 amount, a scale, of which this is a model, was fixed on the Pagoda 

 at Kew, distant 4398*24 feet from the Observatory, and photo- 

 graphed. Each of these plates of the scale, one foot wide, is 

 depicted in the photograph, and occupies rather a large space in the 

 picture the further it is from the optical centre. We were able to 

 measure, by means of the micrometer, the width of the image at the 

 centre, and the width at other parts of the field up to the edge. That 

 gives us the means of ascertaining what allowance ought to be made 

 for distortion, and we can apply this correction and obtain most 

 accurate measurements of the sun. 



I ought to say that the Kevv photo-heliograph had the honour, by 

 its observations in 1 860, of first proving that the luminous prominences 

 which are depicted in this photograph, and which were only to be seen 

 at the period of a total eclipse, absolutely belong to the sun. We do 

 not want that proof now, because we can see them at any time under 

 favourable atmospheric conditions, whether there is an eclipse or not. 



I have already alluded to the transit of Venus. The English, the 

 Russians, and our colonies employed the eleven instruments of the Kew 

 model, before spoken of, but the Americans made use of a long tele- 

 scope, I think nearly forty feet in focal length placed horizontally in the 

 direction of the meridian, and the image was thrown into it by the 



