INVEETEBEATA, CEYPTOGAMIA, MICEOSCOPY, ETC. 781 



heretofore in use, rays from the light or index points have traversed 

 and intersected all these exterior rays or oblique angles in succession 

 to the limit of the field of view, which has been erroneously assigned 

 as angle of aperture." 



Measuring Aperture. — Professor Hamilton Smith in his paper 

 above referred to describes an experiment with a Spencer ^, made when 

 the systems were closed and a small dot of ink put on the flat surface 

 of the front lens, just large enough to cut off the little circle of light 

 that appears when one looks into the objective with the front system 

 toward the eye. Under these circumstances when the objective was 

 attached to the Microscope, not a ray of light could be obtained 

 except what came through the instrument, yet on the sector he was 

 still able to see light when the arm was swung up to 179°. He 

 further says, " The true air angle at the same closed point .... is 

 only 141°." 



Mr. J. Mayall, jun., properly points out * that this experiment is 

 fallacious. " It being admitted that no aperture, properly speaking, 

 can be measured unless the image of a point be rendered, approxi- 

 mately at least, as a point ; does Professor Smith mean to say that 

 when the system of lenses is closed he can measure the true air angle? 

 My experience with immersion lenses of high angle is thab when the 

 system is closed, so as to get definition through the thickest cover- 

 glass and the immersion medium, at that adjustment no true air angle 

 can be obtained. Tlie true air angle can only be actually measured 

 when the objective is adjusted so as to give true focus in air, a focus 

 sensibly free from aberration." 



So far as Professor Smith's apertometer is applicable to immersion 

 lenses, Mr. Mayall considers it is practically the same as Mr. Tolles's 

 traverse lens which yields results equivalent to those obtained with 

 Abbe's aj^ertometer. 



Woodward's Apertometer. j — Colonel Woodward describes an 

 apertometer which he has been using for some time, which is a combi- 

 nation of the Abbe ajjertometer with the well-known sector, and which 

 he thinks has advantages over both that and the Universal apertometer 

 of Professor Smith. 



Colonel Woodward objects to the Abbe apertometer (1) in regard 

 to the cutting away of the surface that corresj)onds to the diameter of 

 the semicylinder at an angle of 45°, rendering necessary the silvered 

 cover-glass with circular central spot, which should correspond to a 

 path of the rays to the oblique surface, and thence by reflection up- 

 wards, just equal to the radius of curvature, an error in selecting this 

 point rendering the reading inaccurate ; and (2) in regard to the 

 graduation into divisions corresponding to an arbitrary scale. The 

 modified instrument is shown in Fig. 17. 



A is a circular disk of brass of about 10 inches radius, inlaid near 

 its circumference with a silver circle divided to sixths of a degree. It 

 is mounted for convenience on a heavy three-legged stool of wood. 



* 'Am. Quart. Micr. Jouru.,' i. (1879) p. 28i. 

 t Ibid., p. 272. 

 VOL. II. 3 F 



