Prof. Dove on several Prism-stereoscopes, 29 



their planes of refraction shall be constantly perpendicular to 

 each other. In the reading of numbers we thus avoid the un- 

 pleasantness of an inversion. 



The application to the measurement of angles of inclination is 

 as follows : — The prisms being set at an angle of 90° in the tele- 

 scope, so that objects appear in their natural position,, the tele- 

 scope is turned round its axis until the vertical cross-wire covers 

 the line, the inclination of which is to be determined; The 

 forward prism is now turned until the vertical wire and the line 

 which it covers coincide with a plummet-line suspended before 

 the telescope. The angle of rotation is equal to half the com- 

 plement of the angle of inclination sought. The reversion-prism 

 can also be placed within the telescope or before the object-glass 

 of the instrument, its size being so chosen that an object can be 

 seen through the prism and through the uncovered portion of 

 the object-glass at the same time. The two lines whose inclosed 

 angle is to be determined are brought to coincidence ; the one 

 seen through the uncovered object-glass, and the other through 

 the prism. If the reversion -prism be set in the interior of the 

 telescope, then the terrestrial instrument is of the same length 

 as the astronomical, the entire length of the terrestrial ocular 

 being thus spared. If in this case it be required that the prism 

 should be capable of being turned round, the tube of the tele- 

 scope may be composed of two parts, one of which fits into the 

 other ; in one part the fixed prism is to be set, and in the other 

 part the rotating prism. 



In all isosceles triangles the condition of total reflexion is ful- 

 filled for those rays which fall parallel to the base and near it. 

 That which has been heretofore affirmed regarding right-angled 

 triangles, is true of isosceles triangles generally. The number 

 of rays, however, which after their first refraction reach the base, 

 decreases as the angle at the vertex becomes more acute. In 

 each particular case, therefore, where another than the right- 

 angled triangle is applied, the angle must be determined which 

 permits of the whole of the rays falling upon the first surface 

 being totally reflected. With an acute angle, the length of the 

 ocular decreases and the light intensity increases. To adjust the 

 prisms, the common method of turning the telescope round its 

 axis, and observing that the position of any chosen point remains 

 fixed, is to be applied. >; -. << "i; '^ 



, 2. Description of several Prism-stereoscopes, and of a simple ^s 

 1 1 ■:. MiiTor-stereoscope. ■. ^' 



When stereoscopic di-awings are executed in white lines upon 

 a black ground, a weak image arising from the reflexion which 

 takes place at the uncovered forward surface of the glass often 



