90 
Transactions of the 
r Monthly Microscopical 
L Journal, Feb. 1, 1869. 
II. — Immersion Objectives and Test-ohjects, 
By John Mayall, jun., F.K.M.S., &c. 
(Read he/ore the Royal Microscopical Society, Octoher 14, 1868.) 
The presentation of Dr. Woodward's photographs of Nobert's 
Nineteen-band Test-plate, brought to the notice of the Society by 
the Hon. Secretary, Mr. Jabez Hogg, affords an opportunity of 
making a few remarks on their vakie as a record of what has been 
done in America in resolving these marvellously fine lines ; and as 
in my experiments I found some difference in the results obtained 
on Nobert's plate by the immersion and the dry objectives, I think 
it will interest the Society to be informed of these results, because 
the relative separating and defining power of the two systems has 
not received that attention which I and many others think it 
deserves. 
The only way we have of verifying the accuracy of the divisions 
of Nobert's Test-lines is by counting them in a measured space. 
For example: if 46 equidistant lines are ruled in the space of 
^-oVrrth of an inch, the interspaces must be at the rate of 90,000 to 
the inch, and so on. An error of one line more or less in counting 
the whole of the lines on such a band would decide the rate of the 
interspaces to be either 92,000 or 88,000, instead of 90,000 to 
the inch. 
Dr. Woodward's photographs support an opinion given by 
Mr. Wenham many years ago, that the time would come when 
photography would reveal minute detail much more palpably than 
it can be seen in the microscope. The reason of this is obvious. 
In photography the object may be illuminated by highly con- 
densed sun-light — a light producing the intensest black shadows, 
and quite unendurable to the eye — and it was with such illumi- 
nation that these photographs were obtained. They may be 
accepted as showing true lines on all the bands to the fifteenth 
inclusive ; but beyond this, they serve little purpose of verification. 
Eepeated trials on the higher bands, as shown in the photographs, 
have convinced me that Dr. Woodward's counting includes or 
rejects such doubtful lines, that it cannot be accepted as exact 
evidence of the number of lines ruled on the plate. I think too 
that he underrates the difficulty of counting the hnes. In all the 
bands beyond the twelfth, as I have seen them in the microscope, 
there is great difficulty to decide on the first and last lines. Dr. 
Woodward seems not to have been sure of the accuracy of the 
count he made on his photograph ; for although in one part of his 
paper in the current (October) number of the Journal of this 
Society, he says the photograph shows the twelfth band as resolved 
