GENERIC IMAGES. 535 



appeared in the " Spectator," of December 28th last, two very inter- 

 esting letters concerning a peculiar form of visualizing possessed by 

 the late Mr. Bidder, the engineer, known in early life as the " calcu- 

 lating boy," and this gift is possessed in a high though less degree by 

 several of his descendants. Thus the eldest son, Mr. George Bidder, 

 Q. C, can mentally multiply fifteen figures by fifteen, though not 

 with the same precision and rapidity as his father. One of the two 

 letters is from Mr. Bidder's friend, Professor Elliot, who writes 

 thus : 



If he saw or heard a number, it seemed permanently photographed in his 

 brain. In like manner he could study a complicated diagram without seeing it, 

 when walking and apparently listening to a friend talking to him on some other 

 subject. The diagraiu stood before him in all its .lines and letters. 



The second letter is from Mr. George Bidder, who writes : 



His memory was of a peculiar cast, in which figures seemed to stereotype 

 themselves without an effort .... (accompanied) by an almost inconceivable 

 rapidity of operation. I speak with some confidence on the former of these 

 faculties, as I possess it to a considerable extent myself (though not to compare 

 with my father). Professor Elliot says he always saw mental pictures of fig- 

 ures and geometrical diagrams. I always do. If I perform a sum mentally, it 

 always proceeds in a visible form in my mind ; indeed, I can conceive no other 

 way possible of doing mental arithmetic. 



Mr. Bidder continues, in a letter addressed to myself : 



If my mind is engaged solving a geometrical problem including the relations 

 of lines, plans, etc., I deliberately build up in my mind a figure, plane or solid, 

 as the case requires; but there is a limit to my power in this respect, e. g., if 

 the problem includes the relative positions and intersections of many surfaces, it 

 becomes a painful effort to grasp them all simultaneously. 



All this shows that mental impressions of extreme vividness may 

 at the same time have great mobility and be subject to " an almost in- 

 conceivable rapidity of operation," and that they need not be fixed in 

 the way that hallucinations often are. 



Next as regards actual blending. Mr. G. Bidder, in very kindly 

 replying to some questions that I put, writes : 



Nothing is easier than to imagine, and to watch mentally, the rotation of 

 anything to which such motion is natural, e. g., a wheel, a crank, etc. In many 

 such cases I incline to think the process consists in calling up a sort of typical 

 image formed out of innumerable bygone experiences. 



This was Mr. Bidder's own view, quite independent of any suggestions 

 from myself, and is therefore all the more valuable. 



The strongest proof that those who have vivid memories of special 

 objects are also capable of blending them is found in the works of 

 such men as Macaulay. I am assured on excellent authority that his 

 visual memory of book, page, and line was of the clearest possible char- 



