INTRODUCTORY RKMARKS. 43 



The first effective achromatic object-glass for a micros- 

 cope was made, under the direction of Dr. Goring, by the 

 late Mr. W. Tulley, in 1824. It consisted of three lenses, a 

 concave inclosed within two convex ones. From its 

 great thickness, when compared with the ordinary object- 

 glass, it received the appellation of the thick* aplanatic 

 object-glass for diverging rays. 



It appears that the celebrated Professor Amici, of 

 Florence, constructed some achromatics about the year 

 1815, but the happy discovery of test objects not being 

 then made, nor the value of angular aperture understood, 

 they were abandoned as useless at that period. In this 

 country Mr. Tulley went on improving the triple form he 

 had adopted ; and notwithstanding he found it extremely 

 difficult to work it on a small scale, and was thus obliged, 

 in order to get magnifying power, to have recourse to a 

 long body and a deep eye-piece, still he made the angle 

 of aperture as large as he well could. The foci of his 

 object-glasses were about 9-10ths of an inch, and 

 the angular aperture 18 degrees. Subsequently he in- 

 creased the angle to 38 degrees, by placing another 

 triple achromatic before it of 6-10ths, so that the acting 

 focus of the combination was only 3-8ths of an inch. 



The objections to this arrangement are, that it is liable 

 to get out of adjustment — that it has twelve reflecting 

 surfaces, the lenses not being cemented together — and 

 that it cannot be worked of very short focus. It is con- 

 siderably easier to obtain any given angle of aperture 



* See a section of it, drawn in plate 3 of the Micrographia. 



