Angular Aperture of Object-glasses. 115 



slit e e, with knife edges, is placed before the object-glass, when the 

 aperture is measured it will cut off all false rays, and confine the 

 image-forming ones to a definite margin. The focus of the object- 

 glass must be exactly in the plane of the front of stop. All rays up 

 to 180^ can then reach this point, and consequently aU true ones 

 within such an angle will be admitted. In this way the angle of 

 Mr. Tolles' ^th was measured. 



In order that no after-quibble might be raised concerning the 

 position of the adjusting collar, this was set to the closest point of 

 the lenses, thus giving the utmost obtainable angle. A brass disk 

 was turned, "OlS in. thick (the exact focal distance of the object- 

 glass). In the centre of this disk there was a conical opening, the 

 diameter of which at the base was much beyond that of fi'ont lens, 

 and at the apex sVth of an inch, coming to a sharp edge. The disk 

 was blackened with perchloride of platinum, and was attached to 

 front of object-glass as follows. It was laid on a glass slip on stage 

 of microscope, with the small end of aperture downwards ; the top 

 side was touched with two opposite minute dots of liquid gum. The 

 object-glass was now brought down gradually, keeping the aperture 

 exactly in the centre by means of the stage movements. This could 

 be done with the greatest nicety ; when the object-glass reached the 

 disk this became attached, and scratches on the glass shp were in 

 focus. 



Tbough the opening at the small end of the aperture was only 

 3ijth, it did not encroach upon the full field of view. There is no risk 

 of injury to the object-glass by the operation, as it outspans and 

 covers it. The object-glass with its attached disk was now tried for 

 angle with the usual sector, still with the lenses at closed point. 

 Instead of 180°, the aperture was at once shown to be the more 

 rational and wholesome angle of 112^,, only six degrees less than the 

 diameter of the lens could possibly admit from the point of focus. 

 With this arrangement the margin of light in the eye-piece was 

 very clear and definite. 



The balsam aperture had now to be ascertained. It is evident 

 that the same som'ce of error from an unshaded front must arise, 

 and is perhaps more hkely to occur, for the reason that the 

 diminished incidences on the surface of the lens facilitate the 

 entrance of false light beyond the angle of true aperture, therefore 

 the shield or cone leading up to the focal point must still be used. 

 I have advocated the tank method for taking immersion apertures, 

 because it requires no focussing or niceties of manipulation, by 

 which errors of accident or design may be attributed, but it is both 

 messy and inconvenient ; I therefore did not adopt it. In some 

 degree this objection applies to Col. Woodward's plan, in which the 

 balsam should be somewhat opalescent, in order to trace distinctly 

 the cone of light that pervades its substance, and with a high-power 



