W' 



8 



fleeted raj would be difFerent from a h', i. e. it would not fall 

 on the speculum in the point b, nor consequently be seen< 

 (by the eye at e), to coincide with C ; but would fall without 

 a b as at d, and would be reflected in do; in which direc- 

 tion the image would be seen, and would be painted in the 

 bottom of the eye, in a different place from that of the di- 

 rect image of C; so that these images would be divaricated ; 

 and it would be necessary to make them unite, by givino- 

 such a motion to the little mirror, as would have made the 

 first reflected ray B E parallel to the incident ray C A , by 

 which the first image would be transferred from vi to c, and 

 the second image from o to the eye at e. 



In the same manner it may be shewn, that if the second 

 reflected ray tended to any point N on the other side of the 

 line B E, from an inclination of the speculum B on the 

 other side, there would be a divarication of the images to 

 the eye at e, till such inclination of the glass was removed. 

 It also appears that the objects C and c, by which the ad- 

 justment is made, may be placed very near the instrument, 

 provided the reflection be made from the middle part only 

 of the glasses, especially of the index-glass, the incidence 

 of rays being difFerent in different parts of it; for unless the 

 sight vane or eye hole for the little mirror, be large, so as 

 that tiie eye could shift across the vane the axis of vision, 

 which ought to be fixed, or the hole be very near the mirror; 

 the image, if reflected from a part of it distant from the 

 middle, would yet not appear coincident with the object seen 



