Diffraction Experiments. 



129 



" gratings/' the revolutions of both gratings (without changing 

 their relative position to each other) gives the appearance of 

 double motion in each cylinder of light spindles ; but if the 

 outer grating is revolved while the inner one is stationary, one 

 cylinder of spindles remains fixed and the other goes round. 



Some experiments with gratings require the power of 

 varying the distance of the source of light. For these Messrs. 

 Home and Thornthwaite have arranged a commodious 

 apparatus, as shown in the cut on the preceding page. 



The next apparatus for diffraction experiments which we 

 shall describe, is the beautiful one exhibited at the scientific 

 soirees of last season, by Messrs. Home and Thornthwaite, and 

 to which allusion was made at the time. This consists of a 

 small telescope mounted on a stand. 



BEIDGE S DIFFEACTION APPABATUS. 



The object-glass is covered by an apparatus devised by 

 Mr. Bridge, by means of which a disc of glass, on which a 

 multitude of diffraction patterns are photographed, is made to 

 revolve in a spiral, so that all the patterns in succession, from 

 the centre to the circumference of the disc, can be brought 

 into the field. The exquisite patterns of these discs were 

 drawn and arranged by Mr. Bridge, and they offer a great 

 variety of different curves, chequers, etc., etc. When the 

 sun is shining, magical and lustrous chromatic effects are 

 obtained by placing on a table or other stand, a dark screen, 

 in the centre of which is a lens of short focus. A mirror 

 behind the screen receives the direct sunlight and directs it 

 upon the lens, as shown in the sketch. An observer a few feet 

 off in front of the lens sees a most brilliant spot of intense light, 

 and if this is focussed by the telescope, and the diffraction disc 

 revolved, a succession of gorgeous effects delights the eye. 

 Stars of greater or less complication, lines of parti-coloured 



