148 FRAGMENTS 01 SCIENCE. 



with great distinctness which vessel contains the largest 

 particles. The retina is very sensitive to differences of 

 light, when, as here, the eye is in comparative darkness, 

 and when the quantities of wave-motion thrown against 

 the retina are small. The larger particles declare them- 

 selves by the greater whiteness of their scattered light. 

 Call now to mind the observation, or effort at observation, 

 made by our President, when he failed to distinguish the 

 particles of mastic in BrUcke's medium, and when you have 

 done so follow me. I permitted a beam of light to act 

 upon a certain vapor. In two minutes the azure appeared, 

 but at the end of fifteen minutes it had not ceased to be 

 azure. After fifteen minutes, for example, its color, and 

 some other phenomena, pronounced it to be a blue of dis- 

 tinctly smaller particles than those sought for in vain by 

 Mr. Huxley. These particles, as already stated, must have 

 been less than 100 1 000 th of an inch in diameter. And now 

 I want you to submit to your imagination the following 

 question : Here are particles which have been growing 

 continually for fifteen minutes, and at the end of that time 

 are demonstrably smaller than those which defied the mi- 

 croscope of Mr. .Huxley : what must have been the size of 

 these particles at the beginning of their groxoth ? What 

 notion can you form of the magnitude of such particles ? 

 The distances of stellar space give us simply a bewildering 

 sense of vastness without leaving any distinct impression 

 on the mind, and the magnitudes with which we have here 

 to do bewilder us equally in the opposite direction. We 

 are dealing with infinitesimals compared with which the 

 test objects of the microscope are literally immense. 



From their pcrviousness to stellar light and other con- 

 siderations, Sir John Herschel drew some startling conclu- 

 sions regarding the density and W'eight of comets. You 

 know that these extraordinary and mysterious bodies some- 

 times throw out tails 100,000,000 of miles in length, and 



