358 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



centrated from below. A stage of mirror glass with a broad rim cemented 

 on shell-wise can be advantageously used. The illuminating mirror 

 Sp can be used on the stage or below it. The prism-carrier Pt can be 

 clamped on the guide-bar aud must be set in the optic axis : it termi- 

 nates in a fork, at whose ends are the bearings for the rotation axis of 

 the prism. The size of the prism depends on that of the front lens of 

 the largest objective used. The hypotenuse plane is silvered to improve 

 the reflection. The prism is set in a metal mounting, and its rotation 

 about its horizontal axis is controlled by the milled head K (fig. G8) 



Fig. 70. 



and limited by the two stop-screws Arr. When the prism has been 

 accurately set, as in fig. 60, with the reflecting plane at 45° to the 

 horizon, by aid of one of the stop-screws, rotation to the other stop 

 turns it through 90° and puts it in the position of fig. 70. The 

 tripod stand is so placed that its feet rest in three prepared spots and 

 is adjusted by means of the two levelling screws. The magnification is 

 estimated in the usual way by means of a magnified glass scale. When 

 the upper surface has been photographed, the stage-holder and prism- 

 holder are undamped, lifted off, and replaced in reversed order : the prism 

 is rotated through 90° and the under side photographed. For the 



