166 



PRACTICAL TREATISE ON THE MICROSCOPE. 



most of the improvements in the achromatic microscopes made 

 in England. 



For aU the detail connected with this important subject, 

 the reader is referred to the paper itself in the Philosophical 

 Transactions, where will be found the various steps by which 

 Mr. Lister was led to such valuable results. The achromatic 

 object-glasses made by our three most eminent opticians, 

 Messrs. Powell, Eoss, and Smith, consist of two or three 

 compound lenses, which are fixed in a long tube or case, those 

 of high power having the adjustment shown in fig. 22, page 

 42, or in section in fig. 116, where A represents the anterior 

 pair of lenses, M the middle, and P the 

 posterior, the three sets combined form 

 the achromatic object-glass. All the 

 first-rate instruments are supplied with 

 as many as five or six of these objec- 

 tives, by which, magnifying powers can 

 be obtained from 20 up to 2,000 dia- 

 meters. The great expense of their 

 manufacture has been always a bar to 

 the more general employment of the 

 Fig. 116. achromatic microscope ; and to diminish 



this, combinations have been made, in which the tube con- 

 taining the front lenses may be drawn off, the back glass 

 being thus used alone for a low power, with a dark stop slid 

 over it. Messrs. Smith and Beck stiU sometimes supply their 

 one-and-a-half inch and two-thirds of an inch glasses so con- 

 structed, and thus produce glasses of three inches and one- 

 and-a-quarter inch; Messrs. Powell and Koss, on the con- 

 trary, always supply distinct combinations. The object- 

 glasses made on the Continent consist of sets of three or 

 more, screwed one upon the other ; of these, the first, second, 

 or third may be used separately or combined ; by this means 

 three difierent powers may be obtained, but their construction 

 does not allow of fine correction throughout the range. A 

 section of a modern compound achromatic microscope, as 

 manufactured by our best makers, is represented by fig. 117, 

 where o is an object, and above it is seen the triple achro- 



