26 Transactions of the Socitty. 



the Microscope : at the top of it there is a right-angled prism 

 (in a box) which reflects the rays along a horizontal tube of con- 

 venient length ; this is crossed at the end by a vertical tube, and 

 at the intersection there is a jiiece of tinted glass which reflects 

 the rays up the vertical tube. In the upper limb of the vertical 

 tube is inserted the eye-piece, and in the lower limb the convex 

 glass which acts as the telescopic object-glass, and the rays from 

 which passing through the tinted glass form an image of the paper 

 in the focus of the eye-piece. As this image is inverted, and it 

 is necessary for easy drawing that it should be erect, an erecting 

 prism is attached below the convex glass. In use the tube, which I 

 have for simplicity called the vertical tube, is inclined, by a motion 

 round the axis of the horizontal tube, to an angle of about 00"^ from 

 the vertical, so that the lower face of the erecting prism becomes 

 nearly horizontal, the paper is put on the table below it and 

 focussed by sliding the object-glass in or out. The light on the 

 object must of course be suitably modified so that the paper and 

 pencil may be distinctly seen. 



I do not put forward this model as the best possible form in 

 which the principle may be applied ; I have no doubt it is sus- 

 ceptible of many improvements, but the principle itself is, I believe, 

 a sound one. It is equally applicable with the necessary modifica- 

 tions to drawing objects in the field of a telescope. 



Lenses may be used to erect the image instead of a prism. 



