366 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
its thickness, so that a circular step A B (figs. 61 and 63) is formed. On 
this step is fixed with mastic a circular glass slide of the same thickness 
as the step (figs. 61 and 63, 0 B). Between the glass slide and the contour 
of the ring rests a circular collar B E (fig. 63) of 4 mm. which carries in 
Fig. 61. Fig. 62. 
