E. M. NELSON ON EVOLUTION OF THE MICROSCOPE. 107 



square. This instrument is figured by G. Adams in Micrographia 

 IllustixUa, 1746, The last time we meet with it is in the 

 second edition of the Essays on the Microscope, by G. Adams,* 

 1798. Here it is made entirely of brass, the legs both above 

 and below the stage highly scrolled — a device which certainly 

 gives more room for manipulating either the object or the 

 mirror. A rack-and-pinion focussing adjustment was sometimes 

 fitted to it by Jones. 



This is the proper place to treat of micrometers, because up to 

 the time we are considering (1738) Leeuwenhoek's method of 

 comparing objects with the bigness of a grain of sand had been 

 employed. The following quotation from Dr. Smith's Opticks, 

 p. 405, will show the progress that had been made on that crude 

 method. "Those that are curious in making exact draughts of 

 the appearance of objects seen in double microscopes may be very 

 much assisted by a lattice made with fine silver wires, or with the 

 strokes of a diamond upon a plane glass, put into the place of the 

 image formed by the object glass ; and by transferring the parts 

 of the object, seen in the squares of the lattice, into corresponding 

 squares of a similar lattice drawn upon paper. It may also be 

 of singular use in philosophical enquiries to know the exact 

 measures of the several vessels and other parts of animal and 

 vegetable substances ; and this, as Mr. Balthasaris has observed 

 in his little treatise upon micrometers, may be done very exactly 

 by a micrometer of the same form as is used in a telescope. For 

 by opening the hairs of the micrometer till they exactly com- 

 prehend an object of a known length, suppose -^^ inch, and by 

 observing the number of revolutions in this opening, the diameter 

 of any other object answering to a known number of revolutions 

 may be found by the golden rule." 



Here we find fully described the most approved methods, which 

 are still in use both for drawing and measuring microscopical 

 objects. 



* With regard to the Adams optical firm, there is a mention of a 

 Nathaniel Adams (optician to H.E.H. Frederic Prince of Wales). This 

 might have been the father of George Adams, who published the Micro- 

 (japliia Illustrata in 1746. Essays on the Microscope, 1787, were written 

 by George Adams (son of the preceding) ; he died in 1795 ; and W. and 

 S. Jones of Holborn, who took over his business, published a second 

 edition in 1797. In this year Dudley Adams, son of George Adams, took 

 out a patent with regard to spectacles. 



