[Footnote 14: When the perspective object is accurately measured by instrument at a known distance from the eye, and the tactual size of the object is also known, the associative distance can be calculated by simple proportion. Multiply the measuring distance by the tactual size and divide the product by the perspective size--the quotient is the distance. The perspective size of objects is greatly exaggerated in realism. Most people think they see a man at his full stature for a distance of fifty yards or so. At that distance the tallest man does not measure half an inch in height. At twenty feet a six-foot man measures 3·6 inches--at ten feet 7·2 inches. The people assembled in a room forty feet long range in real--perspective--height from seven inches to two inches. When a man is nearer than ten feet we do not perceive him in one operation--we observe him in parts which we put together in the mind.] [Footnote 15: Probably Dr. Johnson meant to be humorous in his way. The principles of Idealism are apt to excite mirth in the unphilosophical, but the laugh is not always on the side of the scoffer. A member of the Persian philosophical sect called Samradians once said to his steward: 'The world and its inhabitants have no actual existence; they have merely an ideal being.' The servant on hearing this took the first favourable opportunity to conceal his master's horse, and when he was about to ride brought him an ass with the horse's saddle. When the Samradian asked, 'Where is the horse?'--the servant replied, 'Thou hast been thinking of an idea; there was no horse in being.' The master answered, 'It is true'; he then mounted the ass, and after riding for some time he suddenly dismounted and taking the saddle off the ass's back placed it on the servant's, drawing the girths tightly; and having forced the bridle into his mouth, he mounted him and flogged him along vigorously. The servant in piteous accents exclaimed, 'What is the meaning of this treatment?'--to which the Samradian replied, 'There is no such thing as a whip; it is merely ideal; thou art only thinking of some illusion.' After which the steward repented and restored the horse.