18 THE MICROSCOPICAL NEWS. 
MICROSCOPIC TEST OBJEBGES. 
By E. M. NELsoNn, IN THE Ezglish Mechanic. 
AVING worked at these objects for some years, and having 
also kept pace with the times in objectives and apparatus, I 
give the results of my experience. 1st, the total abolition of oblique 
illumination if one wishes to see the true structure of an object ; 
2nd, object mounted dry on cover. 
I use a P. and L. achromatic condenser, accurately centred to 
the optic axis. The edge of the flame of a:paraffin lamp, with %in. 
wick, exactly focussed on the object, without bull’s-eye or- mirror. 
This illumination, with a P. and L. oil ;4,, N.A. 1°43, easily resolves 
A. Pellucida, dry on cover, with direct light—z.e., without slot or 
stop. 
If .S. gemma is examined by this means, the hemispherule theory 
is at once exploded, and the true structure (which is far more 
beautiful) is revealed. It is something like a most delicate skeleton 
leaf. This, however, is very difficult for a beginner. The 2 for- 
mosum is, perhaps, the best one to try first. Work away at that 
until the hemispheres, which are so easily seen, give place to a 
square grating! To see this, with a 1%, N.A. °74, will severely test 
the lens and the observer’s manipulative skill. A coarse JVav. lyra 
anda Zryblionella punctata, both with square apertures, are very 
easy. If the objective is much out of correction, the square 
apertures will blur round. The next one to try is A. angulatum. 
In this, a fracture should be seen to distinctly pass ‘rough the 
apertures. The apertures will take a rose tint if the glass is 
properly corrected. 
It is manifestly absurd to test an objective by fine diatoms seen 
with oblique light, for only a small portion of a narrow marginal 
zone of the objective is used. The central, and by far the more 
important, part of the glass might be stopped out. 
By the central illumination, however, the whole of the objective 
is used; the centre by the dioptric beam, the margin by the 
diffraction pencils. In former days one used to hear this sort of 
thing said: ‘“ This 51, is a beautiful diatom glass.” ‘This 5 is 
splendid on ‘ Podura,’ but not good at diatom resolving.” (What 
a fine thing for the opticians! One had to buy two glasses, one 
for Podura and the other for diatoms). ' The explanation is very 
simple: For Podura a glass must be good in the centre, and 
for diatoms, with oblique light (the only light used in those days), 
good in the marginal zone. So then the ,,, which was good for 
Podura, and the j, for diatoms, could neither of them have been 
thoroughly corrected from their centres to their margins. I havea 
