196 Transactions of the Society. 



V. — The Lantern Microscope. By Lewis Wright, 



(_Read 12th November, 1884.) 



It is about three years since I was urged by several Fellows of 

 this Society, and others, to turn my attention to the improve- 

 ment of the oxyhydrogon Microscope, being assured that any 

 instrument which would display objects effectivehj even on the 

 scale of (JOO or 700 diameters, would be an immense advance 

 upon anything then obtainable. By one or the other of those 

 interested, all the instruments of most repute were placed in my 

 hands, and it appeared that there was indeed much to be desired ; 

 since not one of them, with the best lime-light possible, would 

 exhibit the bulk of those slides which any demonstrator with a 

 serious purpose in view would desire to place upon the screen. 



My own efforts in this direction were to some extent hindered 

 by my comparative ignorance of microscopy, as such. I do not 

 even yet know, as stated further on, what my own instrument will 

 perform in diatom work, and the problem was to me chiefly an 

 interesting one in optical projection, whose difficulties constituted 

 its main charm. An examination of the instruments lent me, 

 however, presented the clearest internal evidence that previous 

 failure to overcome those difficulties was due to either ignorance or 

 neglect of the peculiar conditions of the oxyhydrogen lime-light. 



The conditions alluded to are easily indicated. If a luminous 

 point is placed at a conjugate focus of a lens, the diverging rays 

 will unite in the other conjugate focus, or parallel rays will be 

 brought to the principal focus. It is obvious, that under such con- 

 ditions, by placing a small lens in various positions on one side or 

 other of this focus, any possible kind of pencil may be obtained, or 

 an illuminated surface of any desired size or minuteness. Ac- 

 cordingly, writers on the lantern Microscope and photo-micrography 

 have described every possible use of a secondary condenser in this 

 way ; so that there is absolutely no room for any really novel (in 

 theory) optical combination. 



Now such a condition fairly represents the case with solar rays, 

 which are sensibly parallel, though even the solar rays cannot be 

 converged to a point. Hence the Duboscq Microscope, the best I 

 have found, and which consists mainly of a small convex lens 

 adjusted by rack and pinion anywhere near the focus of a large 

 condensing lens, will produce good results with the heliostat. It 

 will also produce fair results, though not so good, with the electric 

 arc ; because with this also the radiant point is tolerably small, 

 while the light is ample to allow of waste. But the lime-hght 

 radiant is a luminous surface as large as the thumb-nail, and this 

 quite upsets the whole matter; for the radiant having a very 



