252 OPERATIONS SUBSIDIARY TO INDUCTION. 



In some cases, arbitrary numerical relations are introduced 

 into the definition : thus, a leaf is called bilobate, when it is 

 divided into two parts by a notch ; but if the notch go to the 

 middle of its length, it is bifid; if it go near the base of the 

 leaf, it is bipartite ; if to the base, it is bisect. Thus, too, a 

 pod of a cruciferous plant is a siliqua, if it is four times as long 

 as it is broad, but if it be shorter than this it is a silicula. 

 Such terms being established, the form of the very complex 

 leaf or frond of a fern (Hymenophyllum Wilsoni) is exactly 

 conveyed by the following phrase : ' fronds rigid pinnate, 

 pinnae recurved subunilateral, pinnatifid, the segments lineal- 

 undivided or bifid spinuloso-serrate.' 



" Other characters, as well as form, are conveyed with the 

 like precision : Colour by means of a classified scale of colours. 



This was done with most precision by Werner, 



and his scale of colours is still the most usual standard of 

 naturalists. Werner also introduced a more exact terminology 

 with regard to other characters which are important in mine- 

 ralogy, as lustre, hardness. But Mohs improved upon this 

 step by giving a numerical scale of hardness, in which talc is 



1, gypsum 2, calc spar 3, and so on Some properties, 



as specific gravity, by their definition give at once a numerical 

 measure ; and others, as crystalline form, require a very con- 

 siderable array of mathematical calculation and reasoning, to 

 point out their relations and gradations." 



3. Thus far of Descriptive Terminology, or of the 

 language requisite for placing on record our observation 

 of individual instances. But when we proceed from this 

 to Induction, or rather to that comparison of observed 

 instances which is the preparatory step towards it, we stand 

 in need of an additional and a different sort of general 

 names. 



Whenever, for purposes of Induction, we find it necessary 

 to introduce (in Dr. Whewell's phraseology) some new general 

 conception; that is, whenever the comparison of a set of 

 phenomena leads to the recognition in them of some common 

 circumstance, which, our attention not having been directed 



