SCIOPTICON MANUAL. 193 



last fifty years, and still made by Duboscq, and other 

 French manufacturers, would be the best form ; and, as re- 

 gards the equal distribution of light on the screen this is 

 true, but when such lenses are thus used and of sufficient 

 size to secure this result, their errors of spherical aberration 

 and want of flatness become unendurable. 



We are then fenced in on either side by the necessity of 

 a large and short lens to secure an equal illumination, and 

 the difficulty in securing flatness or correction under these 

 conditions. 



The most successful compromise which we have yet found 

 in this connection is the gas microscope objective, of H-inch 

 focus, made by Mr. J. Zentmayer, the well known manufac- 

 turer of microscopic stands and lenses. 



With one of these, a well defined object, such as a lady- 

 bug, mosquito, or the like, may be thrown on the screen 

 with a clear image, pretty well defined up to the margin, 

 and a field of light so brilliant and regular that it is hardly 

 distinguishable from that of an ordinary magic lantern pro- 

 jecting a colored glass slide of the same object. Of course, 

 with such a power, very minute objects must be rejected, 

 but by a judicious selection a large series of interesting ones 

 can be secured, such as the lady bug or mosquito already 

 mentioned, the ant lion, field spider, and various water 

 insects or Iarva3 of mosquitoes, and the different sorts of 

 flies ; also wood sections, and even objects so small as the 

 eye of a dragon fly, but, above all, with this power may be 

 most successfully shown what are by far the most popular 

 illustrations with the gas microscope, such living specimens 

 as the various larvse above mentioned, and such other 

 things as are to be found in stagnant water. For these the 

 very simple and effective form of life slide, devised by Mr. 

 S. Holman, Actuary of the Franklin Institute, Philadelphia, 

 is invaluable. 



