about one per cent., and it is said that experts can distinguish the difference of a fraction of a degree centigrade by the temperature sense alone. Now while the other senses may distinguish two or more kinds of extension, as, for instance, pitch and loudness in the case of hearing, vision is the only sense with quantitative perception in which the extensions are identical in every respect except in their relation to directions, thus giving a field of vision so-called within which individually different marks may be compared. The eye is capable of judging the coincidence of two abutting lines to one minute of arc, which is a more sensitive determination than can be secured from any other sense perception.
The preceding pages may have conveyed the impression that the study of physics is a stern and difficult one. While there was no wish on the writer's part to magnify the difficulties of this most interesting science, it was a definite part of his plan to show that the proper teaching of physics does not consist in the acquisition by the pupil of first-hand knowledge of phenomena; neither does it consist in trying to implant a spirit of inductive reasoning whereby a student is led to divine the great laws of nature as a discoverer; least of all is the obedient following of directions set down in a manual or given by an instructor the study of physics. That alone is true and successful study which cultivates logical power in dealing with phenomena, gives a tenacious hold upon what is known and adds at least something of how the field in present possession of the science was explored and occupied.