242 
Immersion Objectives and Test-objects. By John 
Mayall, Jun., F.R.M.S., &c. 
(Read before the Royal Microscopical Society, October 14, 1S68.) 
The presentation of Dr. Woodward’s photographs 1 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 value as a 
record of what has been done in America in resolving these 
marvellously fine lines ; and as in my experiments 1 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 rela- 
tive 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 T o ' u 0 th of an inch, the interspace 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 illu- 
minated by highly condensed sun-light — a light producing 
the intensest black shadows, and quite unendurable to 
the eye — and it was with such illumination that these 
photographs were obtained. They may be accepted as 
showing true lines on all the bands to the fifteenth in- 
clusive ; but beyond this, they serve little purpose of veri- 
fication. Repeated trials on the higher bands, as shown 
in the photographs, have convinced me that Dr. Wood- 
ward’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 lines. In all the bands beyond 
1 See Dr. Woodward’s paper in the ‘Journal,’ October, 1868. — Ed. 
