76 DISSERTATION SECOND. [part i. 



VI. In (he next place may be ranked the instances 

 which Bacon calls analogous, or -parallel. These consist 

 of facts, between which an analogy or resemblance is visi- 

 ble in some particulars, notwithstanding great diversity in all 

 the rest. Such are the telescope and microscope, in the 

 works of art, compared with the eye in the works of na- 

 ture. This, indeed, is an analogy which goes much be- 

 yond the mere exterior ; it extends to the internal struc- 

 ture, and to the principle of action, which is the same in 

 the eye and in the telescope, — to the latent schematism, 

 in the language of Bacon, as far as material substance is 

 concerned. It was the experiment of the camera obscura 

 which led to the discovery of the formation of the images 

 of external objects in the bottom of the eye by the action 

 of the crystalline lens, and the other humours of which the 

 eye is formed. 



Among the instances of conformity, those are the most 

 useful which enable us to compare productions of an un- 

 known formation, with similar productions of which the 

 formation is well understood. Such are basalt, and the 

 other trap rocks, compared with the lava thrown out from 

 volcanoes. They have a structure so exactly similar, that 

 it is hardly possible to doubt that their origin is the same, 

 and that they are both produced by the action of subter- 

 raneous fire. There are, however, amid their similarity, 

 some very remarkable differences in the substances which 

 they contain, the trap rocks containing calcareous spar, 

 and the lava never containing any. On the supposition 

 that they are both of igneous origin, is there any circum- 

 stance, in the conditions in which heat may have been ap- 

 plied to them, which can account for this difference? Sir 

 James Hall, in a train of most philosophical and happily 

 contrived experiments, has explained the nature of those 

 conditions, and has shown that the presence of calcareous 



