XXXvI REPORT — 1861. 



the luminous prominences with a piecision impossible of attainment by hand 

 drawings. The measurements which have been made of these prominences 

 by Mr. De la Rue show incontrovertibiy tliat they must belong to the sun, 

 and that they are not produced by the deflection of the sun's light through 

 the valleys ot the moon. The same prominences, except those covered over 

 during the moon's progress, correspond exactly when one negative is laid 

 over the other ; and by copying these by means of a camera, when so placed, 

 a representation is obtained of the whole of the prominences visible during 

 the eclipse in their true relative position. The photographs of the several 

 phases of the eclipse have served to trace out the path of the moon's centre 

 in reference to the sun's centre during the progress of the phenomenon. 

 Now, Rivabellosa being north of the central line of the moon's shadow, the 

 moon's centre did not pass exactly across the sun's centre, but was depressed 

 a little btlow it, so that a little more of the prominences situated on the 

 north (the upper) limb of the sun became visible than would have been the 

 case exactly under the central line, while, on the other hand, a little of those 

 on the southern limb was shut off. It has been proved, by measuring the 

 photographs, that the moon during the totality covered and uncovered the 

 prominences to the extent of about 94" of arc in the direction of her path, 

 and that a prominence situated at a right angle to the path shifted its angular 

 position with respect to the moon's centre by lagging behind .5° 55'. On 

 both the photographs is recorded a prominence, not visible optically, showing 

 that photography can render visible phenomena which without its aid would 

 escape observation. Copies of the two totality pictures are being made to 

 illustrate Mr. De la Rue's paper in the Report of the ' Himalaya ' Expedition 

 by the Astronomer Royal. ; 



Positive enlarged copies of the phases of the eclipse, nine inches in dia- 

 meter, have also been made by means of the camera, and will be exhibited 

 at the Manchester Meeting. 



The Heliograph has since been replaced in the Observatory ; but few 

 opportunities have occurred for using it, in consequence of the pressure of 

 other work ; latterlj', however, Mr. Beckley has been requested to carry on 

 some experiments with the view of ascertaining whether any more details 

 are rendered visible when the full aperture of 3 inches of the telescope is 

 used, than when it is reduced to about one inch and a half. Up to the pre- 

 sent time no definite conclusion can be drawn from the results obtained ; so 

 that, at all events, an increase of aperture does not appear to give a strikingly 

 better result when a picture of the same size is taken with various aperture's 

 of the object-glass. IMore experiments, however, are needed before this 

 point, which is one of some importance in guiding us in the construction of 

 future instruments, can be. answered definitely. Mr. Beckley has obtained 

 sun-pictures of great beauty during the course of these experiments. 



The work of the Kew Observatory is now so increased that it has become 

 absolutely imperative to make some provision for working the Heliograph 

 in a way that will not interfere with the current work of that establishment ; 

 and Mr. De la Rue has been requested by his colleagues of the Kew Com- 

 mittee to take charge of the instrument at his observatorj', where celestial 

 photography is continuously, carried on. This request Mr. De la Rue has 

 kindly acceded to ; and he will for a time undertake to record the sun-spots 

 at Craiiford. as long as it is found not to interfere with his other observations. 

 INIr. De la Rue has contrived, and had made by ^Messrs. Simms at his own 

 expense, an instrument for measuring the photographs, which will much facili- 

 tate the reduction of the results. It consists of a fixed frame in which work 

 two slides, moving at right angles to each other. Each is furnished with a 



