92 
Transactions of the 
r Monthly Microscopical 
Journal, Feb. 1, 1869. 
The advantages mainly claimed for the Immersion objectives 
are : — greater working distance between the object and objective, 
increase of light and superior definition and clearness in the optical 
image, which image is obtained by much simpler illuminating 
apparatus and with less manipulative skill than that considered in- 
dispensable in using high-power dry objectives. 
It is not difficult to see that Amici's system, of connecting the 
objective with the cover-glass by a film of water, very much dimi- 
nishes the reflexion which necessarily takes place on the incidence 
of oblique light when the dry objective is employed. The limiting 
angle of refraction in water being about 48 degrees, it follows that, 
whatever is the degree of obliquity in the incident light on the 
object, the Immersion objective never has to do with rays of greater 
obliquity than 48 degrees. To this, in great measure, is due the 
greater clearness and precision of image obtained. 
Continental opticians and men of science have been aware of the 
merits of the Immersion system during several years past ; and to 
such purpose, that knowing how httle attention it has received here, 
they do not scruple to say that the English no longer take the lead 
either as opticians or as microscopists. 
There are amateurs who make a speciality of declaring that all 
Test-objects can be resolved by some wonderful ^th objective. I am 
told, for instance, that coarse transverse and very fine straight 
longitudinal lines can easily be seen with a ^th on Surirella 
gemma. It is well known that a low-power will show transverse 
lines ; but no objective or method that I know of shows straight 
longitudinal lines on this diatom. The finest images that I have 
seen of Surirella gemma (with Hartnack's -jV th and Nobert's -io^, 
do not confirm Hartnack's opinion that the surface is a series of elon- 
gated hexagons, similar to the drawing in his pamphlet on "Test- 
objects" (published in 1865, and copied by Dr. Carpenter in the last 
edition of his work on "The Microscope"), but show it to be 
analogous to that of the Grammatofhora suhtilissima. Surirella 
gemma may truly be called a touchstone for a high-power objective. 
I do not think that it has been clearly defined as yet. 
Those who profess that so much can be done with ^th objectives 
in resolving Test-objects must not expect higher powers to verify 
the results they obtain. The whole art of testing objectives, I 
imagine, is in being sufficiently critical as to what is a fine image. 
The image shown of Test-objects by a high power of fine definition 
is so much more elaborated as at once to prove that the lower 
powers do not give true definition beyond a moderate magnification. 
For example, a good :^th by Dallmeyer shows transverse lines on 
the Grammatojohora subtilisima; whereas, with equal magnification, 
but with power in the objective (as in Hartnack's -roth) rather than 
in the eye-piece, these transverse lines are seen to be the result of 
