Journal, July 1, 1870. 
20 On the Optical Advantages [oregon 
duce an enormous amount of spherical aberration, when the object- 
glass is tried on the globule test, or in its legitimate use as a micro- 
scope lens, is scarcely perceptible in the telescope arrangement, and 
though a badly-corrected glass may not form an image, yet I have 
no hesitation in affirming that a lens may be made to give perfect 
definition under the latter condition, that will prove utterly worth- 
less as an objective for the microscope. 
IV.—On the Optical Advantages of Immersion Lenses and the 
Use of Deviation Tables for Optical Research. By Roysron- 
Picort, M.A., M.D., M.R.C.P., F.C.P.S., F.R.AS., late Fellow 
of St. Peter’s College, Cambridge. 
Introduction. 
OnE might almost venture to declare that perhaps too much ex- 
clusive attention has been paid to objectives to the neglect of the 
pencils radiating from the object. A brilliant particle, as a podura 
or diatom- spherule, throws out a divergent pencil, which suffers 
extraordinary deviations and reflexions before it is permitted to 
enter the object-glass at all. The infinitely small here escapes our 
attention. But the primary behaviour of the tiny spray of rays is 
of the last importance to the final definition, according to the refrac- 
tions, dispersions and reflexions it undergoes in its primary or nascent 
state, if the term be allowed, of the diverging pencils. Again, im- 
mersed in a refracting fluid and again covered, new transformations 
into fresh strands of the divergent pencils impose new optical con- 
ditions. A large proportion of the rays are absolutely refused ad- 
mittance by the covering glass. But some of these rejected rays are 
at once permitted to pass into the objective by the intervention of 
fluid-films instead of air. 
The tracing of the rays (to scale) for many different selected 
divergent rays emanating from the brilliant particle, and the in- 
vestigation of their behaviour under the different resistances offered 
by interposed media of different deflecting powers, is a problem 
worthy of a great deal of hard labour. The right understanding of 
this primary process, among the innumerable rays just about to enter 
the object-glass, nascent as it were from their very source of light, 
may possibly lead to some new and beautiful results at present 
unexpected. 
The student of these nascent rays, however, must be prepared 
for some laborious work at the logarithmic tables of numbers, and 
sines of the divergent angles, and calculate accurately the mutual 
refractive indices between the different substances and each other. 
