THE MICROSCOPE. 43 



and twenty divisions marked on the screw-head gave mea- 

 sures of the one six-thousandth part of an inch, and hence its 

 use as a micrometer as well as a fine adjustment. In the 

 year 1841, Mr. Powell made another communication to the 

 Transactions of the same society, "On a new way of mounting 

 the compound body of a microscope," a plan which will be 

 again alluded to under the head Compound Microscope ; and 

 in the year 1840, he succeeded in making an achromatic object- 

 glass of one-sixteenth of an inch in focal length, the first that 

 had been seen in this country ; it is in itself a wonderful 

 production, both for delicacy of workmanship and correctness 

 of definition. About this period, his brother-in-law, Mr. 

 P. H. Lealand, who had for some time assisted him in the 

 manufacture of object-glasses, became a partner with Mr. 

 Powell, and from that time up to the present, these opticians 

 have given their undivided attention to the manufacturing, as 

 well as to the improving and perfecting, the optical and 

 mechanical parts of the achromatic compound microscope. 



Mr. Smith, who had been for many years engaged in the 

 manufacture of microscopes of all the ordinary kinds, was Ln 

 1826 employed by Mr. Lister to construct the instrument 

 represented by fig. 21 ; but he did not turn his attention to 

 those of the achromatic form on his own account until 1839, 

 at which time he likewise made object-glasses on Mr. Lister's 

 principles; these, which are of large aperture, were at first 

 constructed on a plan rather different from those of Messrs. 

 Powell and Eoss, the lowest ampUfication was produced by a 

 single achromatic lens, and to increase the magnifying power, 

 another, or, for a stUl higher, a combination of two, was slid 

 over the first. This plan was adopted with the object of 

 furnishing the glasses at a cheaper rate, but more recently 

 Messrs. Smith and Beck make each power a separate com- 

 pound glass, like the others. 



In the year 1841, Mr. Smith was applied to by the council 

 of the Microscopical Society to furnish them with one of his 

 newly constructed achromatic compound microscopes, and on 

 the 24th of November in the same year, the instrument, of 

 which a figiire is given in the second volume of the Micro- 



