EDWARDS. — MANOMETER DEVICE FOR AIR THERMOMETERS. 543 



Scale 



A 



from the lower surface of the plate to the surface of the mercury. This 

 is accomplished by means of the two glass prisms G and II cemented 

 with Canada balsam on the top of 

 the plate. 



In the case of the prism G a 

 beam of light directly from the line is 

 reflected at the surface of the prism 

 and gives an image in the tele- 

 scope D. Seen in the same prism 

 there will be another image some- 

 what above the first and coming by 

 reflection frpm the mercury surface. 

 Measuring the vertical distance be- 

 tween the images by means of a 

 cathetometer telescope we can cal- 

 culate accurately and easily, if the 

 plate and the telescope are horizon- 

 tal, the distance between the glass 

 and the mercury surfaces. The prism 

 H gives a pair of images precisely 

 similar, except that the one coming 

 by reflection from the mercury is now 

 below the direct image. Let b and 

 bi be the vertical displacement of 

 the telescope for each pair of images, 

 d the distance from the line to the mercury surface, x and x^ the 



b by 



values of the ratios - and —, i.e., the effective magnifying powers of 



the prisms and plate. The unac- 

 cented letters refer to prism G, the 

 accented letters to prism H. 



Assuming for the present that the 

 plate and telescope are level, we 

 have, according to Figure 3, a being 

 the angle of the prism and /a the 

 index of refraction of the glass, and 

 the analysis being so simple and 

 obvious as to scarcely need entering 

 here, — 

 /i sin 4 a 



Figure 3. 



X =■ X = - = 

 a 



VI- 



