THE RECENT PROGRESS OF PHYSICAL SCIENCE. 755 



use of our senses presents peciiliar difficulties to scientific investiga- 

 tion. Some kinds of information with regard to their surroundings are 

 of such paramount importance to successive generations of living 

 beings that they have learned to interpret indications which, from a 

 physical point of view, are of the slenderest character. Every day 

 we are in the habit of recognizing, without much difficulty, the quarter 

 from which a sound proceeds, but by what steps we attain that end 

 has not yet been satisfactorily explained. It has been proved that 

 when proper precautions are taken we are unable to distinguish whether 

 a pure tone (as from a vibrating tuning-fork held over a suitable reso- 

 nator) comes to us from in front or from behind. This is what might 

 have been expected from an a priori point of view ; but what would 

 have been expected is that with almost any other sort of sound, from 

 a clap of the hands to the clearest vowel-sound, the discrimination is 

 not only possible but easy and instinctive. In these cases it does not 

 appear how the possession of two ears helps us, though there is some 

 evidence that it does ; and, even when sounds come to us from the 

 right or left, the explanation of the ready discrimination which is then 

 possible with pure tones is not so easy as might at first appear. We 

 should be inclined to think that the sound was heard much more loudly 

 with the ear that is turned toward than with the ear that is turned 

 from it, and that in this way the direction was recognized. But, if 

 we try the experiment, we find that, at any rate with notes near the 

 middle of the musical scale, the difference of loudness is by no means 

 so very great. The wave-lengths of such notes are long enough in 

 relation to the dimensions of the head to forbid the formation of any- 

 thing like a sound shadow in which the averted ear might be sheltered. 



In concluding this imperfect survey of recent progress in physics, I 

 must warn you emphatically that much of great importance has been 

 passed over altogether. I should have liked to speak to you of those 

 far-reaching speculations, especially associated with the name of Max- 

 well, in which light is regarded as a disturbance in an electro-magnetic 

 medium. Indeed, at one time, I had thought of taking the scientific 

 work of Maxwell as the principal theme of this address. But, like 

 most men of genius. Maxwell delighted in questions too obscure and 

 difficult for hasty treatment, and thus much of his work could hardly 

 be considered upon such an occasion as the present. His biography 

 has recently been published, and should be read by all who are inter- 

 ested in science and in scientific men. His many-sided character, the 

 quaintness of his humor, the penetration of his intellect, his simple but 

 deep religious feeling, the affection between son and father, the devo- 

 tion of husband and wife, all combine to form a rare and fascinating 

 picture. To estimate rightly his influence upon the present state of 

 science, we must regard not only the work that he executed himself, 

 important as that was, but also the ideas and the spirit which he com- 



