BCIOPTICON MANUAL. 191 



THE GAS MICROSCOPE. 



BY HENRY MORTON, PH. D. 



The projection of images from microscopic objects directly 

 upon the screen, with the gas microscope, has always been a 

 thing much desired by all those who have made use of the 

 magic lantern as a means of demonstration, but the difficul- 

 ties attending this experiment have been found much more 

 serious than was anticipated beforehand. 



This is especially the case to one who has been accus- 

 tomed to use the solar microscope, in which the advantage 

 offered by the parallelism of the solar rays is of so great 

 value. 



On account of the smallness of the object illuminated, as 

 compared with errors of focalizing or concentration in the 

 cone of rays coming from the condenser, all the advantages 

 in the use of a lens in a magic lantern, as compared with 

 its use in a camera, or the like, disappear, and the lens of 

 the microscopic attachment is left to its own resources (on 

 the] subject here referred to, see Journal of Franklin Iti- 

 stitute, vol. 62, page 208 ; Scientific American, 1863, vol. 

 29, page 163), without any of' that aid from the condensers 

 which they afford so effectively to the objective of the 

 magic lantern in its best form of construction. 



Among the errors which thus become conspicuous, 

 the most manifest and vitally important is the want of 

 "flatness of field." 



By reason of this, while the centre of the image is well- 

 defined, the edges are indistinct and unsatisfactory. To 

 obtain lenses free from this defect has been the continuous 

 effort of some of our ablest opticians for the last ten years, 

 but the success so far has been very limited, and, indeed, it 

 would seem as if the problem was one for whose solution we 

 could hardly hope, for it must be remembered that lenses 



