ADDRESS TO THE ROYAL MICROSCOPICAL SOCIETY. I41I 
causes excellent observers to differ in a most remarkable manner 
about the appearances of the same object under different Micro- 
scopes. 
It has been put very forcibly by Dr. Dippel that if the shape of 
the object is unknown, correction may be a mistake, and that 
when the focus is at a lower plane than the summit of the object, 
correction may positively mislead. 
There is no doubt that an image seen under a certain correc- 
tion, and which is stated to be normal, is modified by under- and 
over-correction. 
How to get at the truth is difficult, except in the instance of 
geometrical bodies and definitely parted lines, but the examination 
of the same object by many observers with different instruments 
gives experience, and without indulging in calculations, including 
the method of least squares, it is finally settled that such and such 
is the real shape. 
The possibility of error remains, however, and it is perfectly 
evident that many a difference of delineation of carefully inves- 
tigated objects results from non-correction and over- and under- 
correction. One cannot but help thinking that the difficulties in 
correcting dry objectives of high amplifying power and great 
numerical aperture, will lead to the almost constant employment 
of immersion objectives. And really the only researches which 
are rendered more difficult by the immersion principle are those 
which have rendered the name of Dr. Dallinger so illustrious. 
There is no doubt that it is impossible to prevent the admixture 
of the medium with the water below the thin cover when minute 
organisms are followed here and there and often close up to the 
edge of the glass cover. 
Amongst the results of not correcting objectives are want of 
definition, haziness, and the production of certain colours, and this 
last phenomenon is often observed in objectives which are corrected 
up to a certain degree and fixed. It is the fashion to correct and 
fix so as to obtain a certain amount of chromatic aberration, a 
ruby tint being considered the best. ‘This is to obviate the effect 
which the perfect achromatism of a glass of large numerical 
aperture has on the eye. 
There can be no doubt that the majority of the recorded 
histology of the minuter structures will have to be worked over 
again with carefully corrected objectives. 
In concluding these remarks on the Microscope itself I must 
enter a protest against the clumsy method of pushing a glass slide 
with a valuable and important object upon it with the fingers, 
under the objective and moving it about. Cheapness of the 
instrument and want of scientific care are the temptations to and 
the causes of this very frequent source of error, which is intensified 
