19 



the former would emerge at a distance from that reflected 

 from its face ; and the interval would to the naked eye ap- 

 pear like a shaded list, preventing the contact of the images 

 observed from being seen by the double reflection, and con- 

 fining the field to one of the images emerging from one sur- 

 face of the glass ; which will be as contracted as above 

 stated.* However, as it will always be easier and more 



* This will readily ai)pear on inspection of Fig. 3 : in which A is the index glass, and 

 B the back horizon glass, placed at right angles to each other ; each glass being Jth ^ 

 of an inch in thickness : on which a beam of light a b, proceeding from a remote object 

 S, is incident on the edge of the mirror A, in an angle with the plane of the edge of about 

 22 degrees, being the complement of the angle of incidence on the same ; which in the 

 quadrant is generally about at least 68 degrees: from which edge it is reflected to the glass 

 B, and reflected again'from both surfaces of the same; the extreme rays a and h of the 

 beam of light, being throughout its progress, distinguished by the same letters ; and 

 those reflected from the back surface marked a 2, and h 2 : their course (as the fig. 

 itself will shew), is traced with sufficient exactness; from which it appears, that the 

 beam of light a b, contracted by reflection from the mirror A to the li^Xh part of an 

 inch in breadth, preserves the same dimension till it enters the eye ; both in the beam 

 *■, reflected from the anterior surface of the glass B, and in the beam 2 reflected from 

 its back surface: for though this latter is diff"used when it has penetrated the surface of 

 the glass, it is again contracted on emerging from it ; and is, as reflected from both sur- 

 faces, become a double and divided beam, the interval between both its parts being al- 

 most the thickness of each of them, which is equal to the sine of 22 degrees to a radius 

 jth of an inch : and if the thickness of the index glass were to that of the horizon glass, 

 as the sine of the refraction of the rays to its cosine, the interval between the beams 

 would be equal to the breadth of either. To fill up the vacuity of the reflected light in 

 this interval, by making the beams x and z issue contiguous, the thickness of the index 

 glass must be to tliat of the horizon glass, as double the sine of refraction, to the cosine : 

 tliis may be made evident as follows. 



Let tiie beam of light a b (fig. 4.), be reflected from the edge of the index glass A t» 



D 2 



