446 SCIENCE PROGRESS 



condensing a number of mental processes. Another illustration 

 of a very different kind may be given. It has been already 

 explained that to guard against mistakes each plate is measured 

 twice over in reversed positions. The two measures of any 

 star are represented by quite different figures, connected by the 

 rule that their sum must represent the whole width of the plate, 

 26'ooo. Thus, if the first measure be 8"352, the second (in the 

 reversed position) should be i7"648; since the sum of these two 

 numbers makes 26'ooo. Now it will be seen that one of these 

 numbers can be derived from the other by the following pro- 

 cesses : subtract 8 from 25 and we get 17; subtract 3 and 5 each 

 from 9 and we get 6 and 4; subtract 2 from 10 and we get 8. 

 This is a straightforward but not very simple mental operation, 

 which most of us would perform for the first time with some 

 wariness. It fell to the lot of one of the computers at Oxford 

 to perform it 'many thousands of times in reading proof-sheets. 

 He presently became so adept that it was easier for him to read 

 the derived figures than the direct ones ! If set to read actual 

 figures before him in the usual way, he would stumble; but 

 allow him to transpose them as above and he proceeded with 

 confidence and accuracy. We know that the picture of external 

 objects which falls on our retinas is inverted, and that neverthe- 

 less there is no consciousness of inversion in our perception of 

 them ; and this result has been ascribed (though not without 

 misconception) to long habit. It was, however, quite new to 

 me to find that the mental process described above could be 

 rendered automatic by the practice of a few months. 



{To be continued) 



