28 Dr. Tyndall on ike Prop-ess of the Physical Sciences : 



Through two prisms situated so that the hypothcnusc edges 

 form a straight line, an ohject appears unchanged in size and 

 shape, but inchned at an angle which is twice as large as that 

 inclosed by the planes of refraction of the two prisms. For it is 

 clear that the double re^ crsion takes place in the same manner, 

 whether the two lines in which the productions of the hypo- 

 thenuse surfaces of the prism cut the object form a right or an 

 acute angle with each other. Everybody will admit>that when a 

 plane is turned 180° round an arbitrary line which lies in it, 

 and then 180° round another arbitrary line which also lies in it, 

 the appeai*ance of the plane as regards shape and size is the 

 same ; but if the lines do not cross each other at a right angle,, 

 the position of the plane will be oblique. 



At whatever stage of the rotation the prisms may stand, if 

 they be both turned together, that is to say, if the case which 

 incloses them be caused to rotate, the image continues fixed. 

 For, as the image of the first prism moves with twice the velo- 

 city of the second prism, the motion, however, being equivalent 

 to a motion of the prism in the opposite direction, the second 

 prism win therefore cany the image back with exactly the same 

 velocity as the first prism carries it forward. 



A system of two such prisms is named by the inventor a 

 reversion^prism. If the reversion -prism be screwed before the 

 ocular of an astronomical telescope, then if the planes of refrac- 

 tion of the prisms stand perpendicular to each other, the tele- 

 scope is converted into a terrestrial one. An ocular so constructed 

 is named by the inventor a terrestrial prism-ocular. The prisms 

 are made fast in a cylindrical collar, the second being capable of 

 rotating while the first remains fixed. The periphery of the 

 piece which turns is divided into degrees like the head of a 

 micrometer-screw ; and upon the cylindrical collar, two strokes 

 which stand opposite to each other denote the position of the 

 plane of refraction of the fixed prism. The zero of the gradua- 

 tion corresponds to the plane of refraction of the moveable prism. 

 When the planes of refraction inclose the angle 0, the telescope 

 is astronomical ; when they inclose an angle of 90°, the telescope 

 is terrestrial. If the planes inclose an acute angle, the observed 

 object will appear turned through twice the angle formed by the 

 planes of refraction, and will remain thus inclined when the tele- 

 scope is turned round its axis. 



Owing to the shortness of the prism-ocular, a terrestrial tele- 

 scope thus constructed is much shorter than the common one, 

 and may therefore be used with advantage in the naval or mili- 

 tary service. It is to be screwed like a shade before the ocular 

 of the astronomical telescope. If it be applied as a terrestrial 

 ocular only, it is best to fasten both prisms together, so that 



