40 AN INTRODUCTION TO SCIENCE 



impressionable connoisseur of seascapes was a one-eyed 

 man. Art cannot deceive the two eyes, because the conflict 

 of their presentations which, but for the sense of touch, 

 would result in confusion, has, as it were, added a new sense 

 of the position and shape of objects. Thanks to the 

 cooperation of eye and hand, we enjoy a sense of tacti-vision 

 which, by long training, we have learnt to exercise without 

 sacrificing the sense of vision pure and simple. We see 

 with the clearness of the lower vertebrates, birds, reptiles 

 and fishes, in which vision is mono-scopic, although we, in 

 common with monkeys and some other of the higher verte- 

 brates, have acquired the power of stereoscopic vision. 



Extension of the Senses by Artificial Aids. Our 

 senses would teach us little of the world in which we live if 

 their capacity for collecting information were not increased 

 by artificial means. By placing a lens or a system of lenses 

 before the eye, the image thrown upon the retina is magni- 

 fied and our power of distinguishing detail proportionately 

 increased. A magnification of 1,000 diameters is equivalent 

 to the subdivision of each "sensational unit" of the retinal 

 surface into 1,000,000. By collecting waves of sound in a 

 concave reflector, their effect upon the drum of the ear is 

 intensified. The microphone renders audible sounds as 

 faint as the footfall of a fly or the beating of a frog's heart. 



More important than the apparatus which has been 

 devised to aid the senses by increasing their power are the 

 instruments which have been invented to take their place 

 instruments which are sensitive to a degree to which no 

 organ of the body, however aided, could attain. Differences 

 of electricity (it seems almost strange in these days that the 

 body is not equipped with any organ which can respond to 

 this mode of motion!) heat, light, colour, weight, chemical 

 reactions of extreme minuteness, are recognised by these 

 instruments of precision and a matter of even greater im- 

 portancethey are registered in a permanent form so that 

 the investigator can refer to them at his leisure. " Science 



