Kffiseprrisei:^^^ PROGRESS OF MICROSCOPICAL SCIENCE. 167 
slides placed in a oxy hydrogen lantern, seven representations of — • 
The Navicula Angulatum, the original object being long, and 
marked by 52,000 striae to the inch. These were photographed upon 
the lantern slides, magnified as follows: 12, 118, 370, 1562, 2344, 
9525, and 19,050 diameters, which gave a series of pictures upon the 
screen, the linear diameters of which were forty times the above, or, 
480, 4720, 14,800, 62,480, 93,760, 381,000, and 762,000 ; the latter 
being in superficial measurement, 762,000 x 762,000, which equals 
the enormous size of 580,644 millions of times that of the original. 
These were particularly interesting, since they exhibited the varied 
appearance of the object when under the different powers, passing from 
what appear to be two sets of oblique parallel lines crossing each 
other, to hexagonal, and finally, under the last to circular markings ;* 
which, however, on moving some distance from the screen, seemed to 
again change into hexagons. The Podura Plumbea Scale was the 
next object. The insect from which this test object is obtained is 
from the to the -^^ of an inch in length, and the scale represented 
in these photographs is the of an inch long, each spike upon this 
being about 4^o^o ^^^^ length. Three specimens were shown, 
which were magnified 522, 756, and 2100 diameters, giving on the 
screen 20,800, 30,240, and 84,000 linear diameters ; these gave most 
satisfactory representations of the spikes, which seemed to be so clear 
and well defined as to leave but little doubt as to their shape and 
structure. The Pleurosigma Formosum, ^^^th of an inch in length, 
and having 36,000 striae to the inch, was x 640 and 2540, or 25,600, 
and 101,600 diameters upon the screen. Finally, to conclude the 
series, the test plate of Nobert — which is regarded as the most 
accurate means of determining defining power — had been photo- 
graphed, and the slides were brought for exhibition. This optician 
has issued these plates with lines gradually increasing in fineness ; 
and his latest works have exceeded any of the former in this respect. 
The first test plate had ten bands, which were ruled at the rates of 
443 — 1964 lines to the millimeter. The second plate, prepared in 
1849, had twelve bands ; the third plate had fifteen bands, the last 
one of which was ruled at the rate of 2216 lines to the millimeter. 
The plates of 1852 had twenty bands, the finest being 2664 to the 
millimeter ; this was described by Mr. Eichard Beck, who, with a ith 
and No. 3 eye-piece x 1300, found thirty-five lines, each about 
Towo English inch apart. Nobert next prepared a thirty- 
band test plate, the thirtieth band of which had 3544 lines to the milli- 
meter. This was the plate described by SuUivant and Wormley.j" The 
last plates made by Nobert have nineteen bands, the 15th corresponding 
to the lines of the 20th band of the 30-band plate, and the 19th ruled 
at the rate of 4430 lines to the millimeter. Dr. Woodward gave a 
somewhat extended description of his investigations in this direction, 
for an account of which the reader is referred to his paper in the 
* "On the Evidence furnished by Photography as to the Nature of the 
Markings on the Pleurosigma Angulatum." By Prof. O. N. Rood, ' Am. Journal 
of Science and Arts,' vol. xxxii., p. 335. 
t ' American Journal of Science and Arts,' January, 1861. 
