OF ANIMALCULES. 23 



angle subtended by it at ten inches from the eye : thus a 

 single lens, which requires the object to be distant from 

 it one inch {i,e. of one inch focus), will magnify ten 

 times in length and breadth, and 100 times in surface : 

 and with respect to a system of lenses, arranged as in 

 the compound Microscope, it will be easy enough to esti- 

 mate their combined power, by first of all referring it to 

 the relative power of a single lens, and then expressing 

 it according to the scale just mentioned. Thus, if an 

 Engiscope were equivalent to a single lens of 1-lOth of 

 an inch focus, we should call the power 100 : if l-20th, 

 200 : l-30th, 300 : &c. &c., always implying linear mea- 

 sure, unless it be otherwise stated. The area or super- 

 ficial magnifying power is obtained by simply multiply- 

 ing the linear measure into itself. 



The Achromatic Engiscope has indisputably prodigious 

 advantages over any other description of Microscope, 

 and particularly that of being equally applicable to a 

 review of many other classes of objects ; this is espe- 

 cially a property in the one I have recently constructed. 

 It affords a luxuriant field of view, and is managed w'ith 

 great facility. If this, however, cannot be obtained, a 

 good Doublet, well put together, possesses sufficient 

 penetration, and a definition scarcely surpassed by any 

 other instrument. The estimable qualities in these mag- 

 nifiers are, that they produce a clear and well-defined 

 outline distance between the object and the anterior 

 lens, and an uniformly colourless, and not a dingy 



