148 NOVUM ORGANUM. 



that which is originally visible, and confers the power of 

 seeing ; and colour as being secondarily visible, and not ca 

 pable of being seen without light, so as to appear a mere 

 image or modification of light. Yet there are instances of 

 alliance in each respect; as in snow when in great quan 

 tities, and in the flame of sulphur ; the one being a colour 

 originally and in itself light, the other a light verging 

 towards colour.* 



36. In the fourteenth rank of prerogative instances, we will 

 place the instances of the Cross, borrowing our metaphor 

 from the crosses erected where two roads meet, to point 

 out the different directions. We are wont also to call them 

 decisive and judicial instances, and in some cases instances 

 of the oracle, and of command. Their nature is as follows. 

 When in investigating any nature the understanding is, as 

 it were, balanced, and uncertain to which of two or more 

 natures the cause of the required nature should be assigned, 

 on account of the frequent and usual concurrence of several 

 natures ; the instances of the cross show that the union of 

 one nature with the required nature is firm and indisso 

 luble, whilst that of the other is unsteady and separable ; 

 by which means the question is decided, and the first is 

 received as the cause, whilst the other is dismissed and 

 rejected. Such instances therefore afford great light, and 

 are of great weight, so that the course of interpretation 

 sometimes terminates and is completed in them. Sometimes, 

 however, they are found amongst the instances already ob 

 served, but they are generally new, being expressly and 

 purposely sought for and applied, and brought to light only 

 by attentive and active diligence. 



For example; let the required nature be the flow and 

 ebb of the sea, which is repeated twice a day, at intervals 

 of six hours between each advance and retreat, with some 

 little difference, agreeing with the motion of the moon. 

 We have here the following cross-ways. 



This motion must be occasioned either by the advancing 

 and the retiring of the sea, like water shaken in a basin, 

 which leaves one side while it washes the other ; or by the 

 rising of the sea from the bottom, and its again subsiding 

 like boiling water. But a doubt arises, to which of these 

 causes we should assign the flow and ebb. If the first 

 assertion be admitted, it follows, that when there is a flood 

 on one side, there must at the same time be an ebb on an- 



* Snow reflects light, but is not a source of light. 



