282 Dwellers of the Sea and Shore 



elucidation. It is well known that a beam of ordinary 

 sunlight actually consists of several different colors 

 which can be shown on a screen by first passing the 

 beam through a glass prism. This color picture is 

 called the spectrum and is composed of visible hues 

 ranging from deep purple through the blues and the 

 yellows to deep red. If, by some suitable arrange- 

 ment, a photographic plate be* placed at the plane of 

 the screen, excluding all other light, an image of the 

 color picture will be recorded. But, in addition to 

 those colors which are visible, there will appear on the 

 plate a record of colors which are invisible to the eye. 

 These colors are at each end of the spectrum, and are 

 known as the infra-red and the ultra-violet rays. No 

 artificial light has yet been devised which eliminates all 

 of these invisible rays. Now, as the light of Noctiluca 

 does not contain them, it will be seen that its illumina- 

 tion is about one hundred per cent efficient, no light- 

 producing energy being lost. However, what value 

 Noctiluca — and, indeed, many another sightless ani- 

 mal which could be named — derives from its power 

 of luminescence is not clear. It is probable that in 

 this instance phosphorescence is an incidental mani- 

 festation of some fundamental cause which has for 

 its end not the production of light but something much 

 more remote from our comprehension. 



So thickly populated with protozoans is the sea that 

 hardly a drop of water collected near the surface will 

 fail to produce numerous individuals. They are, of 

 course, not all liglit givers, like Noctiluca ; but what 

 they lack in this respect they often make up in exquisite- 

 ness of form. Many of them secrete shells, and al- 



