158 



and proceeding from each a tube two inches in length, the 

 one at right angles to the larger or main tube, and the other 

 forming an angle with the lower part of the same of sixty 

 degrees. These projecting tubes are adapted to and should 

 receive a paper or cardboard-tube of about 12-15 inches 

 in length. The mechanical connection of the above with the 

 lamp itself should be such that, on elevation or depression 

 of the lamp, the whole arrangement should move at the same 

 time and in unison. Such adaptation and arrangement 

 virtually is that exhibited by me at a previous meeting oi 

 the society, and is manufactured by Westmon, of Centre 

 Street, 



By this conversion of the source of illumination into, as 

 it were, a dark chamber, with simply an outlet or conduit 

 for the passage of the rays to the mirror or prism, you avoid 

 not only the lighting up of the room with rays thrown from 

 your lamp in every direction, and thus receiving upon the 

 retina a mass of confused impressions Avhich interfere greatly 

 with observation (and in this connection it will be at once 

 seen that the use of a hood attached to the eye-piece with a 

 similar view, always disagreeable and inconvenient to the 

 microscopist, is removed), but in addition are saved all trouble 

 of so moving or manoeuvring your lamp as to avoid the 

 accident of incident rays upon the object, an occurrence 

 totally destructive of and annihilating all attempts at defi- 

 nition or resolution. 



If the edge of a flat flame as preferred by some is desired, 

 by means of a diaphragm, with slit, affixed to the inner sur- 

 face of the metallic chimney, this can be obtained. For 

 regulating the intensity of the illumination, the use of the 

 diaphragm placed beneath the stage for such purpose by the 

 makers is not to be recommended. The alteration of the 

 distance of the lamj) from the mirror or prism, inasmuch as 

 the intensity of light issuing from a luminous point diminishes 

 in the same proportion as the square of the distance from 

 such point increases, will be found a preferable way of 

 effecting the object. Dark-ground illumination, a variety 

 merely of oblique in which only rays emanating from the 

 object are acted upon — the illumination of opaque objects, 

 and illumination in connection with the polariscope, I must 

 pass over or reserve the consideration of for possibly another 

 paper, as I fear I have already trespassed too far upon your 

 patience and time. 



