144 Scientific Proceedings, Royal Dublin Society. 



this pair of lenses being exactly equal to the radius of the circular 

 scale ; and they are so placed in the box that the optical centre of 

 the pair is in exact coincidence with the centre of the box. Behind 

 this is placed a right-angled prism which reflects the rays in a 

 vertical direction on to a piece of parallel glass, p, hinged to the 

 outside of the lid, and by which the rays are reflected in a 

 horizontal direction into the eye of the observer. 



This piece of parallel glass, which folds down on the lid for 

 convenience of packing, is, in its working position, placed at an 

 angle of about forty-five degrees to the lid, and is coated with 

 the same film of sulphide of lead that I use for gun-sights. This 

 film has the property of reflecting nearly half the light which falls 

 upon it, and of transmitting the other half. 



The result is that the eye, placed as shown in the figure, sees the 

 opposite landscape through the glass, and also a part of the scale 

 projected upon the landscape. 



The pencils of light entering the eye from the scale being 

 parallelized by the pair of convex lenses, the scale appears to be in 

 the plane of the object ; therefore, there is no parallax, and 

 whether the eye be moved, or the lid of the box turned for the 

 purpose of taking the bearings of various objects, the scale always 

 appears immovable as respects the object, so long as the lower box 

 is not moved. Again, as the focus of the pair of lenses is equal to 

 the radius of the scale, every space between the divisions of the 

 scale represents its true angular value on the horizon. Con- 

 sequently no index is required, every object having its bearing 

 projected upon it. 



The instrument is mounted on a tripod stand, with parallel 

 plates for levelling, and a rather stiff vertical joint. 



To take bearings of various objects as regards each other, the 

 observer turns the lid round until he sees the object through the 

 leaded glass ; but he need not set this with exactitude ; if he can 

 see the subject anywhere in the field,- it suffices. He then, holding 

 the lid with one hand, turns the lower part of the box with the 

 other hand, till the of the scale corresponds with his object. 

 Then he turns the upper box or lid round, so as to see each 

 object seriatim ; and he will find the bearing of each object, as 

 respects the first, projected upon it. He can then read off these 

 bearings as fast as an assistant can write them down. 



