VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 365 



well centered) be divided into 2 equal parts or segments, by cutting it straight 

 through the centre ; and let a piece of machinery be so contrived, as to hold 

 these two segments in the same position to each other, as they stood in before 

 they were cut asunder ; and to be capable at the same time of drawing them to 

 different distances from that position, in the manner as represented in fig. 

 7, pi. 8. 



Each of those segments will form a distinct image of any object, to which 

 they are directed ; differing in nothing from that which might have been made 

 by the whole glass before it was cut, except in brightness. And while these 

 segments are held in their original position, the images will coincide, and be- 

 come one single image as at first ; but in proportion as they are drawn off from 

 that situation, the images will separate more or less, according to the distance 

 they are drawn to. By this means the images of two different objects, or of 

 different parts of the same object, not very far from each other, may be brought 

 to a contact or coincidence at the focus : and this coincidence may be viewed to a 

 very great nicety with a proper eye-glass. 



The measure of the angle subtended by the two objects, whose images are 

 thus brought to a coincidence, depends on 3 things; 1st, a careful observation 

 of the coincidence of the images : 2dly, an exact measure of the distance, which 

 the glasses are drawn out to, from that situation which makes the image single : 

 and lastly, a true knowledge of the focal distance of the glass. How the angle is 

 to be found from these measures, and how it may likewise be come at, by view- 

 ing two land-objects at a convenient distance, will be shown hereafter in the ex- 

 planation of the figure. It is easy to understand, in the mean time, that the 

 angle will be measured with more accuracy, in proportion to the length of the 

 glass which is used for that purpose ; but the difficulty of managing long teles- 

 copes is no less apparent. Therefore the most practicable method of using this 

 micrometer to advantage, is to apply the divided object-glass to the object-encj of 

 a reflecting telescope : for, as the apertures of this sort of telescopes are large 

 in proportion to their lengths, they will admit of very long glasses ; nor will the 

 measures be any way afiected by the metals or glasses, which the reflector is 

 composed of: and the angles will be found in the same manner, as though the 

 images were viewed with a single eye-glass, in the manner of a common re- 

 fracting astronomical telescope ; but with this advantage, that as the images will 

 be exhibited larger and distincter by the reflecting telescope ; and as every part 

 of it will be much more manageable than a long refracting telescope ; so the 

 contact or coincidence of the images will be more accurately observed. 



Explanation of the Figure. 

 The two semicircles represent the two segments of the object-glass, whose 



