74 TRANSACTIONS OF THE [pDEc. 12, 
plainly seen, and the angles of reflection and refraction are 
measured. It is readily observed, moreover, that, as theangle of 
incidence increases from 0° to 90°, the ratio of the quantity of 
reflected light to refracted light increases. ‘Turning still further, 
the beam from the lantern strikes normally on the curved face 
and is totally reflected within the glass at its plane face. Turn- 
ing now still further, the reappearance of a refracted beam an- 
nounces the critical angle, which is read on the circle. 
he special advantages of this very simple apparatus are as 
follows : 
I. No new adjustments of the beam are required, however 
the angle of incidence may be varied. 
II. The high index of refraction of glass causes more noticea- 
ble deviation than when water is used. 
III. The beam of light, both before and after division, is 
better defined. 
IV. The necessary expense is scarcely anything more than 
that of grinding the hemicylinder of glass. 
In every particular this is more convenient than any other 
apparatus for the same purpose than the writer has seen. 
For the suggestion of a hemicylinder the writer acknowledges 
his indebtedness to an old note-book belonging to his friend, 
Prof. D. G. Eaton, who did not have the apparatus made. 
Pror. D. 8. Martin spoke in commendation of the appa- 
ratus, as one of great simplicity and elegance. 
Pror. W. P. TRowsBripGe and PRESIDENT NEWBERRY pre- 
sented observations on 
