44 GENERAL HISTORY OF 



necessarily obliged to speak of my own productions, and 

 even to praise them, however objectionable it may seem, 

 and repugnant to my own feelings ; but I have done so, 

 because firmly persuaded that it will be serviceable to 

 many persons, and this, I am sure, will be received as a 

 sufficient apology for intruding myself into this work. 

 In order to draw a fair comparison between different in- 

 struments, I have, at great cost, possessed myself of the 

 best object glasses of all the foreign artists of note, and 

 having most patiently and carefully made trial of their 

 efficiency, no doubt rests upon my mind as to the re- 

 lative qualities of each. Of the mechanical part of the 

 continental microscopes I have elsewhere expressed an 

 opinion. 



The magnifying powers of a complete microscope for 

 perfectly examining all kinds of Infusoria should range 

 from 50 to 1000 diameters; and as this cannot be obtained 

 with first-rate glasses, without recourse being had to 

 several sets, such an instrument would be necessarily 

 expensive. The microscope first mentioned has two or 

 three sets of glasses, varying from 35 to 500; and the 

 second, one set, from 80 to 300 diameters ; so that, as 

 before stated, all the most interesting observations on 

 Infusoria may be conducted with either of these instru- 

 ments, whilst additional sets may be obtained as occasion 

 requires. 



It is important to notice, that in all cases where the 

 magnifying powers of microscopes are spoken of, the 

 standard of sight used in computing them should be 

 known, otherwise very erroneous ideas will be formed. 

 In all my publications, from 1827 up to the present time, 



