130 Diffraction Experiments. 



light in beautiful curves and angles appear in succession, and 

 the astonishment is great to notice what kind of diffraction 

 grating gives rise to such unexpected effects. If the princi- 

 ples explained at the commencement of this article have been 

 apprehended, some notion may be formed of the way in which 

 each dark space of a grating may be the occasion of interferences 

 between waves of light spreading laterally and meeting with 

 other waves that have traversed longer or shorter paths. 



To use Mr. Bridge's apparatus at night, an electric or oxy- 

 hydrogen lamp is necessary to develope the full splendour of the 

 effects. If magnesium wire becomes cheap enough, it would 

 do admirably, and be always ready. . Some notion, though a 

 feeble one, may be gained by the employment of an ordinary 

 lamp ; but those who have not the command of powerful 

 sources of artificial illumination must be content to experiment 

 on sunshiny days. 



Some pleasing and instructive effects can be produced by 

 placing a photographed diffraction pattern near the eye at one 

 end of a tube, and at the other a stop with a small central hole, 

 or a slit, or a plurality of holes, arranged in a pattern such as a 

 triangle, or as the shape of an italic 8. The gorgeous colours 

 do not appear in these experiments, and the pattern seen by the 

 eye is much simpler than when the telescope and night source 

 of light are employed. It is, however, well to study the 

 difference of the effects produced by the two methods, and 

 Messrs. Home and Thomthwaite supply the means to those 

 who require it. If a photographed diffraction pattern is placed 

 at the eye end of a microscope tube, the small stop of an achro- 

 matic condenser will do for a source of illumination at the 

 bottom. 



Mr. Bridge's apparatus gives the most splendid appearances 

 when a sufficiently brilliant light can be obtained. The 

 ' ' Diffraction Kaleidoscope" has the advantage, that for its very 

 important though more limited operations, it is always ready, 

 and, like the ordinary Kaleidoscope, may be passed from hand 

 to hand and pointed without trouble at any ordinary source of 

 light. Other experiments are conveniently performed by aid 

 of the other apparatus figured above. 



