ANGULAR APERTU11E. 



evident, therefore, that by the mere effect of this increased illumi- 

 nation, the lineaments showing the structure of the object, which 

 were altogether imperceptible in c and D, began to be developed 

 but very imperfectly in E, were more visible in F, and became 

 quite distinct in G. 



The great and manifest importance, therefore, of the angle of 

 aperture to the efficiency of the microscope, renders it indispen- 

 sable that easy and practicable means should always be attainable 

 for determining it. If the distance of the object from the object- 

 glass, and the virtual opening or diameter of the object-glass 

 could be always exactly measured ; and if all the rays which 

 fall on the object-glass could be assumed to pass through it, and 

 to converge upon the image, then the angular aperture would be 

 an element of very easy calculation. But it is not practicable to 

 obtain these data, and it cannot be assumed that all the rays 

 which are incident upon the object-glass will pass through it, 

 and be made to converge upon the image. 



16. Instead, therefore, of calculating the angular aperture in 

 this manner, it is determined by immediate experiment. 



The greatest angle of aperture of which a given lens is capable, 

 will be found by determining the greatest obliquity with which 

 it is possible for rays to fall upon the object-glass, so as to be 

 refracted by it to the eye-glass. The following method of ascer- 

 taining this, for any given object-glass, was contrived and 

 practised by Mr. Pritchard, at an early epoch in the pro- 

 gress of the improvement of the microscope, when the importance 

 of the angular aperture was demonstrated by that eminent artist 

 and Dr. Goring. The same method, with but little modification, 

 is that still practised by opticians. 



Let m m, fig. 9, be the microscope, the object end being fixed 

 upon a pivot, so that the eye end can be moved over a graduated 

 semicircle. Let a small luminous object, such as the flame 

 of a candle, be placed in the direction r r, at the distance of 6 or 

 8 feet, so that the rays proceeding from it to the object-glass may 

 be considered as parallel. If the microscope be directed towards 

 the candle, all the rays will fall perpendicularly on the object- 

 glass, and will evidently pass through it to the eye-glass. If the 

 microscope be then turned on the pivot to the left, the rays will 

 fall more and more obliquely on the object-glass, and a less and 

 less number of them will pass to the eye-glass. 



When such a position as m m is given to the microscope, those 

 rays only which fall upon the border of the object-glass upon the 

 right of the observer, will arrive at the eye-glass, and the field of 

 view will then appear, as shown at /, half illuminated and half 

 dark. If the microscope be moved beyond this position, the field 



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