328 ON LIGHT. 



almost exactly equidistant, exceedingly ' minute, non- 

 reflective grooves, and when, if held close to the eye, a 

 candle is seen in it reflected, its image accompanied 

 with two lateral and very vivid spectra of similar origin, 

 and an impression of the surface taken on black seal- 

 ing-wax presents the same phaenomenon. 



(in.) When, as occasionally happens, the eyes are 

 suffiised with a nebulous film (due to the presence in the 

 lacrymatory secretion of extremely minute globular par- 

 ticles of equal size), the image of a candle in a dark 

 room some feet distant is seen surrounded with two or 

 three broad circular halos of rainbow colours alternately 

 ruddy and green. Similar halos are formed round the 

 candle when viewed through two i^ieces of clear glass 

 between which has been placed a little oil mixed with 

 the dehcate powder of the common puff-ball or lycopcr- 

 don, reduced to a thin even film by pressure and gently 

 rubbing them together. In this case they are much 

 more vivid and beautiful, the tints being those of the 

 colours of thin plates beginning from the centre, only 

 more dilute, so that it is difiicult to discern more than 

 a feeble indication of the fourth ring. Their diameters, 

 however, unlike those of the coloured rings figured in 

 Fig. 6, increase in arithmetical progression, or nearly so; 

 that of the first or smallest, reckoned to the minimum 

 of illumination, being 21 36' or thereabouts. They owe 

 their origin to the exceeding minuteness, uniformity of 

 size and sphericity (or at least circularity) of outline (if 

 flat discs) of the spores of this fungus. 



(112.) The explanation of these and of other ph?eno- 



