IS 



middle part ofi the back horizon glass. * By the glass herein- 

 after proposed to be used for Mr. Blair's adjustment, instead 

 of the polished edge of the index glass, the beam of light is 

 reflected to the eye undivided, which Avill allow the axis of 

 vision to pass through the axis of the back horizon glass ; as 

 it ought to do, whether for adjustment of this glass, or for 

 taking angles ; and as the axis of vision cannot be the same 



* Tliis glass lies so obliqueto the eye, that I thiak it yet remains to be enquired what 

 is to be considered as its middle part, whether the middle of the fore or back surface 

 or the middle of its substance, or lastly that ppiut in the Siinie, -which is the vertex of 

 tiie angle made by the incident ray with the sanre refracted by its fore surface after re- 

 flection from its silvered surface* . It would appear to me of litlle moment, which of 

 the two beams of light, proceeding, singly from the middle of the index-glass, and re_ 

 fleeted double from the two surfaces of the horizon glass, be chosen for adjustment as the 

 fixed axis of vision, (for both cannot be iudiscriminatejy used, as emerging from dif- 

 ferent parts of the glass,) provided the reflected image be seen only by the same beam, , 

 issuing from the same part of the horizon glass in all reversals of the quadrant ; were it 

 not that the axis of vision ought to pass through the middle or axis of the glass, for 

 the convenience of direct as well as reflex vision ; according to which the reflected 

 ray cannot, in the oblique position of the glass, impinge on the middle of either sur- 

 face ; but must be made (by turning the instrument in its plane, and placing the sight 

 vane properly,) to fall on its fore surface between the middle of it, and the edge next 

 the eye, if the reflection is to be made from the fore surface; and between the middle 

 and the remote edge, if the image reflected from the back surface is to be seen. The 

 proper place for reflection in the designed axis of vision, may be marked on the 

 face of the glass, by sticking to it a fine waxed thread; and thea the black line on the 

 card before mentioned, covering the face of the index^lass, (or such another thread 

 fixed along the middle of it,) must be made to coincide with this thread in every posi- 

 tion of the quadrant for this adjustment; and as two images of the line will appear from 

 the two surfaces, one only of them must be invariably used : the card to be removed im. 

 order to view the objects, when the line on it is made coincident with the thread. 



