313 CORRESPONDENCE. 
put in over the small drum lens, the mirror is worked until it gives a 
reflexion of the lamp—strong on the edge of its diameter farthest from 
the lamp. 
I use as lamp, and most strongly recommend the following simple 
plan. On the sliding tube which carries the shade of an Acland 
lamp, I slide a tin shade blackened, covering the whole chimney. In 
this shade is a circular aperture opposite the flame, against which I 
fix an ordinary condensing lens; a tin cap slips over this with an 
aperture } inch in diameter, at the base of a cone projecting 2? inches ; 
at the mouth 1} inch in diameter. I have two small cones to pass 
into this, as diaphragms;-one with small circular aperture, the 
other a slit the size of the flame. I find it best to blacken all parts 
of the tinwork. I have given Mr. Curteis a drawing of the arrange- 
ment. 
I have, since I wrote to you, tested the power of the instrument in 
every possible way; it has never failed me. I can get diatoms bril- 
liantly illuminated on perfect black ground, and on any shade up to 
a soft grey light. With the 4 inch of Ross, the Type-Platten is a 
most beautiful study. With the highest objectives, I, with ease, get 
all the definition of which they are capable, with very little trouble. 
Let me add, I have arrived at the conclusion there are some— 
daily I read of more—individuals whose eyes must be constructed 
after a fashion very different from my own, and that of anyone I ever 
met with. Their crystalline lenses must have been by nature gifted 
with a power of correction which counteracts any and every short- 
coming in any object-glass. To them “the screw-collar” is a super- 
fluity ; they have a special visual power which adapts itself to the 
thickness of any covering glass. They have a peculiar power of 
penetration by their own eye-piece which supplements in the most 
extraordinary way the eye-pieces of the best opticians; to resolve a 
Pellucida into hemispheres with a + inch of Ross is an easy thing! 
IT must confess I envy these most highly gifted men. I have 
worked hard at Diatomacese with the best powers of the best makers. 
I have used good, expensive sub-stage illumination, as well as some 
months’ work with my own contrivance, I can make nothing of 
Pellucida beyond the beauty of its outline; I can with ease make out 
the longitudinal and transverse lines of the larger Rhomboides, with 
their intersection ; the small Rhomboides with some care gives me one 
set of lines well, the other faintly ; Crassinervis beats me. 
The best tests of good magnifying power with penetration for high 
powers, are, to my mind, good specimens of Versicolor and A. trilin- 
gulatus ; an object-glass which shows the full structure of these beautiful 
objects, I hold to be next to perfect, especially when seen on a black 
background. I shall be most glad if any of my brother microscopists 
are more successful than I have been with the Exhibitor; I shall be 
quite content if they meet with my own success. Nature has only 
given me the ordinary eye, now old; younger men of gifted eyes, 
may, for all I know, spot Pellucida with the Exhibitor; I shall hear 
of it with satisfaction, yet with envy. 
S. G. Osporne. 
