486 PHILOSOPHICAL TRANSACTIONS. [aNNOI731. 



hemorrhages from the stomach, depending on too much heat and impetus 

 of the blood, and occurring in young subjects; yet Dr. M. is far from recom- 

 mending the use of iced Hquors in cases of flooding after parturition, or in 

 hemorrhages which occur in persons who, having weak stomachs and bowels, 

 are subject to flatulency and indigestion. 



The Description of a new Instrument for taking Jingles. By John Hadley, 

 Esq. Fice Pr. R. S. Communicated to the Society on May 13, 1731. 

 N°420, p. 147. 



This instrument is designed to be of use, where the motion of the objects, 

 or any circumstance occasioning an unsteadmess in the common instruments, 

 renders the observations difficult or uncertain. 



Its contrivance is founded on this obvious principle in catoptrics; that if the 

 rays of light diverging from, or converging to any point, be reflected by a plane 

 polished surface, they will, after the reflection, diverge from, or converge to 

 another point on the opposite side of that surface, at the same distance from it 

 as the first; and that a line perpendicular to the surface passing through one of 

 those pomts, will pass through both. Hence it follows, that if the rays of 

 light, emitted from any point of an object, be successively reflected from two 

 such polished surfaces, that then a third plane, perpendicular to them both, 

 passing through the emitting point, will also pass through each of its two suc- 

 cessive images made by the reflections; all three points will be at equal distances 

 from the common intersection of the three planes; and if two lines be drawn 

 through that common intersection, one from the original point in the object, 

 the other from that image of it which is made by the second reflection; they 

 will comprehend an angle double to that of the inclination of the two polished 

 surfaces. 



Let RFH and rgi, fig. 1, pi. 13, represent the sections of the plane of the 

 figure by the polished surfaces of the two specula bc and de, erected perpendi- 

 cularly on it, meeting in r, which will be the point where their common sec- 

 tion, perpendicular also to the same plane, passes it, and hri is the angle of 

 their inclination. Let av be a ray of light from any point of an object a, fall- 

 ing on the point f, of the first speculum bc, and thence reflected into the line 

 FG.. and at the point g of the second speculum de, reflected again into the line 

 gk; produce gf and kg backwards to m and n, the two successive representa- 

 tions of the point a; and draw ra, rm, and rn. 



Since the point a is in the plane of the scheme, the point m will be so also 

 by the known laws of catoptrics. The line fm is equal to fa, and the angle 



