8 
Proceedings of the Royal Irish Academy. 
film of water, to be placed on the covering glass of the object; and it 
is found that definition, working distance, light, and magnifying 
power are much improved thereby ; these object glasses are called im- 
mersion lenses, or hydro-objectives. N'ow, it is the purpose of this 
communication to introduce the same principle of immersion to be ap- 
plied to the illumination of microscopic objects, whereby corresponding 
advantages will, I believe, be found to accrue. 
Having thus stated the object of this Paper, it would seem that 
little further need be said, the details of the application of the prin- 
ciple of immersion illumination, and the advantages arising, being 
almost self-evident, I think the Academy at large will take sufficient 
interest in the enumeration of these details and advantages to permit 
me to occupy their time a little longer. 
If we regard the best way of viewing objects by unassisted vision, 
we shall easily arrive at the conclusion that an object is best viewed 
where it is illuminated by diffused light — that is, where the light 
reaches the object from all azimuths and altitudes ; does not impinge 
on the retina, except from the object itself; and where the light is 
sufficient, and not too bright. Parallel rays of light give rise to dis- 
tui^bing shadows, which may lead to erroneous conceptions of the true 
character of the object; and light concentrated from one quarter or 
side introduces errors of the same character, although not to the same 
extent. It was a belief that the same manner of illumination which 
is found to be best adapted to unassisted vision would also be found to 
be that best suited for microscopic objects that led me to examine the 
subject more carefully. 
Ordinarily, when we look into a microscope, we feel disposed to 
shrink from the sudden glare of light which floods the field, contracts the 
pupil of the eye, and in time injures the retina, this glare prevents us at 
first from seeing anything whatever ; presently, we begin to perceive 
a something, transparent in parts — in fact, the object under the micro- 
scope is rendered visible by the relative opacity and transparency of its 
parts, the shadowswhich the more opaque parts cast on others and on the 
eye, and the caustics of light which the highly refracting and reflecting 
portions of the object throw on those of different density. ISTow, all 
these effects are injurious to the recognition of the true aspect of the 
surface and structure of an object, and must lead to erroneous impres- 
sions — in proof of this I may refer to the many, and various, and dif- 
ferent interpretations of minute forms put forward by microscopists, 
arising more from defective modes of illumination than from errors on 
the part of the observers. 
Mr. Grubb, and the late Mr. Bergin, long ago perceived the value of 
properly regulated illumination ; and Mr. Grubb' s stand is considered 
to be one of the most perfect, where side light is admitted ; but if 
shadows be considered to be injurious to correct vision, much time and 
shifting of the object and light will be required to obtain a true in- 
terpretation of what is seen. For lined objects, the microscope of Mr. 
Grubb leaves little to be desired in an instrument, and London and 
