STANDARDS OF COMPARISON. 31 



the earth's centre, and the surface of the earth is not at a 

 uniform distance from its centre ; so that slight fractional 

 variations take place with every trifling change of locality : 

 and the yard measure, in the same way, varies from the 

 standard yard to a fractional degree more or less whenever 

 it is in a temperature different from that agreed upon for the 

 standard yard ; for all bodies contract and expand under the 

 influence of temperature, and temperature itself is equally 

 defective in its certainty and equally dependent on an 

 arranged and conventional standard, such as the various 

 thermometers Fahrenheit's, Centigrade, Wedgwood's, &c. 

 The same thing is equally true of all the other obvious 

 qualities of bodies, as a little reflection will make 

 abundantly plain. Diversity of colour, for example, is 

 found to be relative and subjective to the effect of those 

 complementary colours on each other which when com- 

 bined make white light ; . for a faint white light appears 

 green near an intense red light, and blue near an intense 

 yellow light, &c., so that all the qualities of bodies are 

 relative ; form, as we have said, being alone excepted. A 

 square is a square, a triangle a triangle, and a circle a 

 circle, &c., whatever be their size ; for form is not 

 dependent upon size, but on proportion of parts ; and this 

 is the only peculiarity or characteristic of bodies which 

 the eye or the touch in their normal health can make us 

 absolutely certain of without calling in the aid of com- 

 parison. 



Bishop Berkeley, in his Work on the Eye, tells us that 

 the situation of an object is determined only with respect 

 to the objects of the same sense; in other words, the 

 touch perceives the position of an object with reference 

 only to other objects touched, and the eye perceives the 

 position of an object seen only with reference to the 

 position of other objects seen ; and this is strictly correct 

 when applied to the question of erect vision, and within 



