IMAGES AND SHADOWS. 



665 



IMAGES AND SHADOWS. 



By W. J. YOUMANS, M. D. 



SO much has been said lately of the wonders of spectrum analysis, 

 that we are very apt to forget the other and equally marvelous 

 properties of the agent by which it is produced. Spectrum analysis 

 is a rare and curious experiment, but the more familiar effects of light 

 which we daily experience are really just as wonderful, if we will but 

 pause to reflect upon them. Science means knowledge, and the 

 science of optics embodies our knowledge of light ; but how much, 

 after all, do we know of it ? A great deal, undoubtedly, of its modes 

 of action, but very little, if any thing, of its nature. We have an hy- 

 pothesis or supposition about it, and work out ingenious conclusions, 

 logically and experimentally, and say that they are proved ; but how 

 far are we from comprehending them. 



That light moves at an amazing velocity, is shown in several 

 ways ; and all the methods bring ns to about the same results which 

 are expressed in numbers and are demonstrably true ; but what finite 

 mind can enter into the meaning of the statement that the luminous 

 ray moves forward at the rate of 185,000 miles in a second of time ? 

 Between the two ticks of a second's pendulum, we are told that light 



Fig. 1. 



Rectilinear Propagation of Light. 



would pass round our globe seven and a half times. But who has a 

 notion even of the dimensions of our globe? The number of thou- 

 sands of miles through it and around it have been calculated, and the 

 calculations harmonize with the whole body of astronomical knowl- 

 edge ; but we can form no adequate conception of such magnitudes. 

 We patch together different shreds of our mental experience of large 



