756 Transactions of the Society. 



travelling back through the objective in a path widely different 

 from that of the visual pencil." 



During a recent visit to the Conservatoire des Arts et Metiers 

 at Paris, I saw the device here described, attached to the Micro- 

 scopes of the comparator, with which the operations of the French 

 Section of the International Bureau of Weights and Measures are 

 conducted. Its introduction is due to M. Tresca, who has used it 

 since 1871. It is possible that the invention by M. Tresca may 

 have been prior to this date. 



Subsequently, during a visit to the establishment of Troughton 

 and Simms, at Charlton, I mentioned to 31 r. Simms that I had 

 made use of this form of illumination in the Microscopes of 

 the meridian circle of Harvard College Observatory, thereby 

 securing far better definition and nearly ten times the magnifying 

 power. After a moment's search Mr. Simms produced an illumi- 

 nator of exactly the form described by Mr. Tolles and by Professor 

 Wild, which he had constructed as early as 1869, at the instance 

 of Mr, Warner, a retired gentleman residing at Sussex Place, 

 Brighton. 



According to the present evidence, the priority of publication, 

 and, I believe, of invention also, must be assigned to Mr. Tolles. 

 Without doubt M. Tresca was the first to make an actual use of 

 this method of illumination in exact measurements. 



The objective of which a sectional view is given in Fig. 73, was 

 made for me by Mr. Tolles, with special reference to its adapta- 

 tion to the examination of the divisions of the copper- platinum 

 metre of the X form which M. Tresca did me the kindness 

 to trace. It has an aperture of 30° and a focal power of 1 inch. 

 The front system of lenses is at A. The back system is at 

 B. A rectangular prism, whose surfaces c, d, e, are ground and 

 polished, is shown entering one side of the mounting, immediately 

 above the front lens. Parallel rays of light entering at c pass into 

 the prism, are reflected from d, emerge at e, impinge upon the 

 front lens A, and have their principal focus at F ; the focus of the 

 objective being at F', where the object is seen. The light having 

 its focus at F is better distributed on account of the greater 

 breadth of the pencil at F'. It might be sui)posed that if a con- 

 denser were applied to the prism, the light thereby being brought 

 to a focus at F', a better illumination would be secured. In actual 

 experience, Mr. Tolles has found that this is not the case. 



The prism is held in place by a spiral spring pressing upon a 

 ring which fits rather loosely upon the tube. By means of the 

 screw at / any required inclination can be given to the prism. The 

 field of illumination can be regulated by pushing in or withdrawing 

 the prism. When it is entirely withdrawn, the objective takes the 

 ordinary form. 



