Bemarks on the Aperture of Object-glasses. 269 



latum or the striae of Grammatophora subtilissima beneath a 

 covering glass one seventy-fifth of an inch thick (by actual measure- 

 ment). It is fair to say too that this power of working through a 

 thick covering glass with good definition is possessed in a high 

 degree by both the T Vth and ^th immersion objectives of Mr. 

 Tolles, described by me in former papers in this Journal. I note, 

 for instance, that both these glasses will work with good definition 

 through covers of the thickness just mentioned, which none of 

 the T Vths, T Vths, or iths, and no other high-angled ith in the 

 Museum collection, will do.* 



Having determined, then, that I ought to measure the angle 

 when the combination was closed, and having satisfied myself that 

 the sector was not to be trusted under the circumstances, I devised 

 the following plan, which may be commended for its simplicity and 

 for the definite character of the results. 



I had long used an easy mode of measuring the angles of 

 objectives in air, which is, in fact, a modification of the plan of Dr. 

 Eobinson, so justly commended by Mr. Wenham.t I screw the 

 objective into a tube which pierces the shutter of my dark room, 

 the back of the objective being towards the light, and I throw 

 through it, by means of a solar mirror, a parallel pencil of sunlight, 

 which, of course, is brought to a focus in front of the lens and 

 crosses, forming a cone of light. By adjusting a white cardboard 

 protractor horizontally in the middle of the cone with its centre at 

 the visible focus, I measure at once, and without the necessity of 

 any calculation, such as was proposed by Dr. Eobinson, the angle 

 of the pencil which crosses at the principal focus ; and this angle, as 

 Dr. Eobinson has correctly shown, is not materially greater than 

 the angle which would be formed if the light radiated from the 

 conjugate focus used to obtain distinct vision with the eye-piece at 

 the extremity of the microscope body. 



To measure the angle in balsam on the same principle, I 

 simply made a thin tank rather more than three inches square, by 

 filling with hot balsam the space between two sheets of plate-glass 

 held about the sixth of an inch apart by narrow strips of glass on 

 three slides. When the balsam had cooled I had, of course, a laye** 

 of solid balsam of the size of the tank, with one side open. The 

 tank was carefully levelled horizontally in the cone of light, as the 

 cardboard protractor had been, and a drop of fluid balsam on the 

 side where the solid balsam was exposed served to make contact 



* I may remark here that the thickness of cover through which an objective 

 will work is not limited by its aperture, though this limits the working distance 

 on uncovered objects, but by the extent to which the motion of its posterior com- 

 binations neutralize the increasing aberration jjroduced by increasing thickness 

 of cover. The character given to the posterior combinations by the maker deter- 

 mine the available limit in each case. 



t 'Monthly Microscopical Journal,' November, 1872, p. 233. See also 'Pro- 

 ceedings of the Royal Irish Academy,' vol. vi., p. 38, 1854. 



