18 Obyject-glasses and their Definition, — [Month's Microscopical 
III.— Object-glasses and their Definition. 
By F. H. Wenuam, Vice-President R.M.S. 
As this subject is now under discussion, I again venture to trespass 
on these pages with a few remarks. If I incline to defend our 
highest powers against the assertion that “ the best of them pro- 
duce spherical aberrations,” it is not because I am disposed to let 
well alone, and rest under the personality of a “ satisfied optician,” 
but on the plea that both the assumed bead structure of the Podura, 
and also the optical means resorted to for demonstrating such an 
error, have failed in proving its existence. 
By the use of the plane mirror and sunlight, Colonel Dr. Wood- 
ward at once cleverly hits upon the illumination that affords an 
analysis of this beaded appearance, and considers it as an interference 
phenomenon. ‘That this gentleman possesses object-glasses of the 
first quality, with the most unrivalled skill in adjusting and using 
them, cannot be questioned, as his superb photographs of difficult test- 
objects testify. Having seen the beaded appearance, his extreme 
diffidence about obtainmg a photograph of it is not very encourag- 
ing for the advocates of this structure, and Dr. W. suggests that 
Dr. Maddox be induced to undertake the task. If this structure is 
to be generally admitted it will be by the aid of the skill of these 
gentlemen, but I much doubt whether it can be accomplished, for 
there is a truthfulness in the photographs of these tests that scarcely 
admits of a deception. The most difficult markings will come out, 
almost in spite of the illumination, which does not even require that 
nicety of adjustment sometimes so tedious in ordinary observation. 
In November, 1854, I read a paper “On Microscopic Photo- 
graphy ” before the Microscopical Society. I had previously obtained 
impressions of several test-objects, and exhibited one of the Angula- 
tum magnified 15,000 diameters, and stated at the time that it “shows 
the configuration of the markings perfectly black and distinct, in a 
far greater degree than we can ever hope to see them through the 
compound microscope, and it is my opinion that if ever the structure 
of these difficult tests is to be proved it will be by the aid of photo- 
graphy.” This prediction has since been verified by the labours of 
Drs. Maddox and Woodward. 
The assertion that “if a blaze of light is sent through the mi- 
croscope, false appearances are exhibited,” must need some qualifica- 
tion. I have condensed the rays from the clear meridian sun, first 
with a bull’s-eye three inches in diameter, and then through an 
achromatic condenser, upon the Rhomboides and other difficult tests, 
which are thus enveloped in a fearful blaze of light and heat, but 
gloriously does the picture emerge at last with the most delicate 
markings shown on a screen 10 feet distant. This is unquestion- 
