Correspondence. 237 



CORRESPONDENCE. 



The following letter is from the Rev. J. D. La Touche : — 

 " Shadows during tiie Solar Eclipse. 



" It has often been remarked that the light during an eclipse of 

 the sun is of a very peculiar character, differing not only from the 

 diffused light of a hazy day, but from that of a less luminous body, 

 such as the moon. This arises, I believe, to a great extent, from 

 the peculiar nature of the shadows thrown by all objects in conse- 

 quence of the great change which has taken place in the shape of 

 the surface from which the light proceeds. 



" My attention was attracted to this during the late eclipse, when, 

 in order to give a large party of school-children simultaneously a 

 view of the wonderful sight, without the aid of smoked glass, I made 

 a few small holes in a piece of card, and holding this at some dis- 

 tance from a sheet of white paper, just as many beautiful crescents 

 appeared, exhibiting to my much-pleased audience, the progress of 

 the eclipse. 



" In making this experiment, however, it was soon manifest that 

 not only the image of the sun, transmitted through the little holes 

 in the card, was changed from the ordinary circle (to which, by the 

 way, we are so accustomed, that I dare say many, when they behold 

 it, are not aware that it is really an image of the sun itself), but it 

 appeared that the edges of the shadow of the card had also under- 

 gone an equally remarkable change. 



" Every one must have observed, that around the edges of every 

 shadow cast by the sun, is a gradually softened-off margin or penum- 

 bra, which, when the object casting it is at six feet distance from 

 that on which it falls, extends, under ordinary circumstances, for 

 about nine-sixteenths of an inch equally round it on every side. This 

 is caused by the sun being, not a luminous point, but a disc of con- 

 siderable size, every portion of which gives off rays of light. Where, 

 as in the case of the electric light, the luminous body is a mere point, 

 all shadows will be sharp and distinct — even those of hairs will be 

 defined with a clearness which strikes most spectators with surprise ; 

 but where the diameter of the luminous surface is considerable, a 

 certain breadth of half-shadow is produced, exactly corresponding to 

 the angle which the disc subtends, and the distance of the opaque 

 body which casts the shadow from the screen on which it is received, 

 and this penumbra will necessarily be modified by the shape of the 

 luminous disc as well. Thus, when the sun is undergoing eclipse, 

 its crescent form will alter the shape of every shadow it gives rise 

 to. In our direction, the diameter of its disc remains unaltered, 

 while in the other it is greatly diminished, causing a corresponding 

 change in the marginal shadow of every object. 



" Now, in the late eclipse, at the time of greatest obscuration, the 

 crescent was in a somewhat horizontal position, and accordingly, 

 though all vertical shadows remained of the same width as before, 

 horizontal ones were greatly diminished, and this was very striking 

 in our experiment. \Vhen the piece of card, moreover, was caused to 

 turn round in its own plane, a regular proportional change took 



