804 
Transactions of the 
t Monthly Microscopical 
Journal, Dec. 1, 1869. 
I wisTi to add here that Mr. Browning pointed out to me that 
the heading of the Podura was rather unequal, some of the heads 
heing larger than others. This is exceptionally the case ; hut I 
have found in numerous ohservations a great regularity in the size 
of the heads on the same scale. 
They are most perfectly seen when the axis of the cone of light 
coincides with that of the ohjective, and the cone of light from the 
radiant is of very small aperture. 
Extract from a Letter to the President. 
October l.~I feel firmly convinced that within a few months 
the Podura headings, such as I have descrihed them, will he 
thoroughly estahhshed. I purposely delayed puhhshing my 
results in 1862, hoping that further advances might he made in 
improving definition, and this has unquestionahly been done by the 
immersion lens, which I have used this year with a Powell and 
Lealand's reth. With this lens I have been able to confirm the 
observations of former years in scales and objects of extreme diffi- 
culty. Before using this lens, I had succeeded in gaining a new 
intensity of definition in the dry way, and in balancing the uncom- 
pensated residuary aberrations ; and I have used the Podura scale 
as a very exquisite test of the lenticular corrections, though there 
are other tests of a higher order still. 
I have endeavoured also to improve the definition of the immer- 
sion lens by extraneous compensation. 
Beck gives a very beautiful steel engraving of the test-scale 
under 1200 diameters. The spines are precise, and exhibited, as 
he saw them, and as thousands still see them, and as Colonel Wood- 
ward photographs them actinically with Powell and Lealand's i^ih. 
and sVth. But very curiously, in the middle of Beck's work, there 
is an engraving of the Podura, described " out of focus,'' being a 
series of parallel bands, which Mr. Aldous has drawn for me with 
the camera lucida under 4000 and 2500 diameters. These bands, 
as I see them, are wholly composed of beads of the diameter of the 
band, the under-beads being out of sight. 
The extraordinary difierence between the performance of the 
Hydro-objective and of the Pneumo-ohjective (the plate of air or water 
making enormous differences in the aberrations of the glasses) must 
make it apparent to ordinary common sense that our old-fashioned 
glasses are wrong somewhere, and if not in failing to converge the 
image of a point to another point, I know not where to find it, ^. e. 
in aberration, — chromatic aberration being more easily compensated. 
I know it is very difficult to throw aside the creed and belief of 
forty years, and I have hesitated a long time to bring forward my 
views, being perfectly convinced that there would be a hattle of 
