250 A LIBRARIAN'S WORK. [xn. 



would thus be impracticable to place our final re- 

 liance on any other arrangement than an alphabetical 

 one, it by no means follows that a subsidiary subject- 

 catalogue is not extremely useful. He who knows 

 that he wants Lloyd's book on the undulatory theory 

 is somewhat more learned in the literature of optics 

 than the majority of those who consult libraries. 

 For one who knows as much as this, there are 

 twenty who know only that they want to get some 

 book about the undulatory theory. Now a subject- 

 catalogue is pre-eminently useful in instructing such 

 people in the literature of the subject they are 

 studying. They have only to open a drawer that is 

 labelled "OPTICS," and run along the cards until 

 they come to a division marked " OPTICS Wave- 

 T/ieory" and there they will find perhaps a dozen 

 or fifty titles of books, pamphlets, review articles, 

 and memoirs of learned societies, all bearing on 

 their subject, and enabling them to look it up with 

 a minimum of bibliographical trouble. Such a 

 classified catalogue immeasurably increases the use- 

 fulness of a library to the general public. At the 

 same time, the skilful classification of books presents 

 so many difficulties and requires so much scientific 

 and literary training that it adds greatly to the 

 labour of catalogue-making. For this reason great 



