1879.] 



On a Direct Vision Spectroscope. 



483 



prism and transmits it in a plane at right angles to the axis of the 

 collimator to the second prism. 



This second prisim is moveable abont an axis parallel to its edge 

 and to the axis of the telescope, and has a right angled reflecting 

 prism attached to it, so that the light after traversing this prism twice 

 passes the second time through the fixed prism and so by reflection 

 into the telescope. The lever carrying the second prism with its 

 reflecting prism is moved by a micrometer screw, by the head of 

 which the movement of the prism is read. 



We placed the design in the hands of Mr. Hilger, some time since, 

 and we now exhibit the instrument to the Society. 



In the last number of the " Journal de Physique," M. Thollon 

 describes some modification of his instrument, but it does not seem 

 that his modified plan is so well adapted to the ordinary use of a 

 chemical laboratory as ours. 



The accompanying diagram represents a section through the prisms 

 at right angles to the axis of the collimator and telescope. 



April 24, 1879. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The Right Hon. Richard Assheton Cross, Secretary of State for t he 

 Home Department, was admitted into the Society. 



The following Papers were read : — 



