Neiv Apertometer. 275 



we accept these serious inconveniences, the microscopist who uses 

 this apparatus has no convenient means of verifying the correct- 

 ness of the scale as ruled by the maker, but must always accept 

 it as correct. 



These considerations induced me to modify, for my own use, 

 the apparatus of Abbe, and I have accordingly constructed the in- 

 strument shown in Fig. 11. 



y^ is a circular disc 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 black walnut. I had this graduated disc made a full circle, 

 because I intended to use it for another purpose also. Of course 

 a graduated semicircle of the same size would have answered 

 equally well. Into a hole in the center of this brass disc a pin is 

 fitted on which swings the radial arm (j5) which carries, on one 

 side of the center of rotation, the body of a microscope (^), while 

 its extremity is provided with a vernier, clamp (Z>) and tangent 

 screw (C). On the other side of the central pin the radial 

 arm carries a table, on which is mounted a semicircle of crown- 

 glass (i^) of about two inches radius and half an inch thick. This 

 is so mounted that the edge, which corresponds to the diameter of 

 the semicircle, is directly over the center of rotation, and the mi- 

 croscope objective can be focussed exactly upon the center of the 

 semicircle. At this spot a thin glass cover, silvered except at a 

 central circular hole (or vertical slit) about -^^ of an inch in diame- 

 ter, is cemented with Canada balsam, the central hole (or slit) 

 being fitted precisely over the center of the semicircle. A suit- 

 able achromatic convex lens (a four-inch objective answers very 

 well) is screwed at the end of the draw-tube of the microscope 

 body, and serves to convert it into a telescope, precisely as in the 

 apparatus of Abbe ; and, indeed, this apparatus can be used pre- 

 cisely like his. For this purpose I have had the semicircle en- 

 graved to degrees, and two shutters or indices provided, so that I 

 can use it in this way when so disposed ; but it is not in this man- 

 ner that I prefer to employ it. 



The particular method for which the apparatus was constructed, 

 and by which exact measurements can be made, is as follows : 

 Using the microscope as a telescope, I view some distant 

 object so small that it only occupies an extremely minute 

 portion of the field, and so bright that it can easily be discerned. 

 For this purpose I prefer the slit of a spectroscope placed at 



