On the Size lest adapted for Achromatic Glasses. 125 



object appears : the field of view is proportionately large, and 

 imiforml V good and distinct, and the motion of the objects passing 

 it proportionately less: thus they may be observed with greater 

 ease and quiet attention. But here it may be well to observe, 

 tlierc is no use in the pencil of rays being of larger diameter 

 than the optic pupil, which is commonly calculated at one-tenth 

 of an inch, varying in magnitude according to the brightness or 

 obscurity of the object presented to it. The natural state ap- 

 pears to be that of dilatation ; and the contraction, a state of 

 violence produced by an effort originating in the mind : when 

 the light is too strong, or the object too bright, we contract the 

 pupil to intercept thai excess of light which would injure the 

 eye: when the light is faint, the pupil expands, that a greater 

 quantity of light may enter the eye, and thus make a stronger 

 impression upon it. This contraction and dilatation of the pupil 

 may be observed by holding a looking-glass (or, what is still bet- 

 ter, the lowest small speculum of a Gregorian telescope) before 

 the eye at a window, and turning gradually from it, continually 

 looking at the eye. It may be more easily and perfectly seen 

 by attentively watching the eye of another, I think it is most 

 agreeably observed in a fine full blue eve. 



To ascertain the magnifying power of a telescope, if we measure 

 the diameter of the aperture of the objoct-glass or speculum, and 

 that of the little image of it which is formed at the end of the 

 eye-piece, the projiortion between these will give the ratio of 

 the magnifying power. To measure the diameter of the pencil 

 of rays with great ease and accuracy, Mr. Ramsden about the 

 year 177'T contrived a clever little instrument, which he called 

 a dynanieter; for though, when single lenses are used, the power 

 of a glass is readily discovered by dividing the focal length of 

 the object-glass by that of the eye-glass, — in eve-})ieces of the 

 common construction, especially those of a negative focus, it is 

 very difficult to measure in this manner ; nor can it be done with 

 any accuracy with those eye-pieces which are rr.ade for erect 

 vision with four eye-glasses. 



The dynanieter is principally composed of a fine micrometer 

 screw, and a divided plano-convex glass; by means of which the 

 image of the pencil of rays is completely separated, and the dia- 

 meter of it known to the greatest nicety. The wheel or head 

 of the micrometer is divided into a hundred equal parts, and a 

 figure engraven over every fifth division, which is cut rather 

 longer than the others ; 1, 2, 3, and so on to 20: but adding an 

 to each figure in calculating, it will then read oflf,' 10, 20, 30, 

 and so on to 200. The nonius is divided into 15, 10, towards 

 U, and 5 on the contrary side. 



The revolutions of the micrometer head will bring the edge of 



the 



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