70 DISSERTATION SECOND. [part i 



I. The first place in this classification is assigned to 

 what are called instantiae solitariae, which are either ex- 

 amples of the same quality existing in two bodies, which 

 have nothing in common but that quality, or of a quality 

 differing in two bodies, which are in all other respects the 

 same. In the first instance, the bodies differ in all things 

 but one; in the second, they agree in all but one. The 

 hypotheses that in either case can be entertained, concern- 

 ing the cause or form of the said quality, are reduced to a 

 small number ; for, in the first, they can involve none of 

 the things in which the bodies differ; and, in the second, 

 none of those in which they agree. 



Thus, of the cause or form of colour now inquired into, 

 instantiae solitariae are found in crystals, prisms of glass, 

 drops of dew, which occasionally exhibit colour, and yet 

 have nothing in common with the stones, flowers, and me- 

 tals, which possess colour permanently, except the colour 

 itself. Hence Bacon concludes, that colour is nothing 

 else than a modification of the rays of light, produced, in 

 the first case, by the different degrees of incidence ; and, in 

 the second, by the texture or constitution of the surfaces 

 of bodies. He may be considered as very fortunate in 

 fixing on these examples, for it was by means of them that 

 Newton afterwards found out the composition of light. 



Of the second kind of instantiae solitariae, Bacon men- 

 tions the white or coloured veins which occur in limestone 

 or marble, and yet hardly differ in substance or in struc- 

 ture from the ground of the stone. He concludes, very 

 justly, from this, that colour has not much to do with the 

 essential properties of body. 



II. The instantiae migrantes exhibit some nature or 

 property of body, passing from one condition to another, 

 either from less to greater, or from greater to less; arriving 



