122 Diffraction Experiments. 



season, nor do they hybernate, "but, on the contrary, are in a 

 much better condition for microscopic observation than during 

 the summer. The forms which chiefly occur are most of them 

 of large dimensions, and living, as they appear to do, princi- 

 pally upon the transparent glair produced from decomposing 

 vegetables, exhibit a clearness and translucency which is un- 

 usual at other seasons, and hence they may be fed with carmine 

 or indigo under peculiarly favourable circumstances. In the 

 Naides, the Planarise, the Rotifera, and the Entomostraca, this 

 glassy transparency is still more remarkable. Their alimentary 

 canal appears completely emptied of all opaque particles, and 

 their bodies are entirely destitute of oil globules, so that the 

 details of their anatomy are traceable with extraordinary 

 clearness, and moreover, the frequent occurrence among them 

 of strange-looking creatures, evidently larval conditions of 

 other forms of life, is well calculated to stimulate the curiosity 

 of the observer whose time and opportunities permit him to 

 enjoy the pleasure of watching their development. 



DIFFRACTION EXPERIMENTS. 



Theee are no experiments in optics more brilliant and beautiful 

 than those which illustrate the effects produced by what is 

 called the Diffraction of Light, a term which etymologically indi- 

 cates a bending of the rays out of their course, and also a cer- 

 tain division, or separation of them. The name is not perhaps 

 very happily chosen, and M. Babinet in explaining certain 

 phenomena of diffraction uses the term paragenie, or lateral 

 propagation. The precise nature of the action that takes place 

 in cases of diffraction can only be understood by those who are 

 capable of following the mathematical illustrations given in such 

 works as Sir John Herschers Treatise on Light. It is, however, 

 possible to explain enough of the matter to give the general 

 reader a notion of the subject, correct as far as it goes, 

 and which, without attempting to elucidate the intricacies 

 of the question, will augment the interest to be derived 

 from contemplating a series of easily obtained and splendid 

 effects. 



First, let us take from Sir J. Herschel a diagram showing 

 what occurs when a ray of light strikes against the edge of an 

 opaque object, and also quote from him a passage describing 

 the action. He says: "When an object is placed in a very 

 small beam of light, or in the cone of rays diverging from an 

 extremely small point, such as a sunbeam admitted through a 



