OP ARTS AND SCIENCES. 139 



tween the objectives of the two microscopes. I will compare the action 

 of the objective of Oberhaeuser to the manner in which a plano-con- 

 vex lens treats the rays of light which pass through it, from any object. 

 Those rays which pass near its axis are brought to a focus at the 

 farthermost possible point from the lens, whilst the rays which pass 

 through the periphery are converged at a much nearer point, and be- 

 tween the axis and periphery there are all degrees of convergence. 

 The difference between the farthermost and nearest points of conver- 

 gence may represent the distance or depth through which the objective 

 takes cognizance of things, and will accounf for the fact that I saw all 

 the rings of the Pine-dot at one time. 



" The action of the objective of Spencer's microscope may be 

 compared to that of a parabolic lens, which converges all the rays 

 of light to one absolute plane, and therefore forms what is called a 

 flat Jield. 



" Now out of this field, either above or below its horizon, it is not 

 possible to see anything, and on this account, when the innermost ring 

 (B, C, a) of the dot was in view, the others were not to be observed ; 

 and when the field was lowered to the second ring (h), the innermost 

 one (a), being above the horizon of the field, was invisible; and, again, 

 when the outermost and lowest ring (c) was reached, the middle one 

 (6) also vanished. 



" Were this outermost ring as distinct as the others, it might have 

 been possible to detect its relations by means of the Oberhaeuser ; 

 but since it is the exceedingly delicate, reverted edge of the per- 

 foration, the narrow aperture of this ordinary objective does not 

 admit sufficiently oblique rays to define it, to say nothing of its 

 being confused with the other rings which are in view at the same 

 time. 



" I would here remark, that this peculiar structure is most frequently 

 to be observed in old wood, when the cell-wall (B, y') has also become 

 perforated, and even has retreated from the deposit layer as far back 

 as the edge of the lenticular interspace. In young wood the perfora- 

 tion corresponds with the figures usually given. I have used this dis- 

 covery, not only to show how little may be understood of the structure 

 of a familiar and much treated of body, but also as a preliminary illus- 

 tration of the exceeding value of a flat field and a wide angle of aper- 

 ture in microscopic investigations. 



