Markings of the English Gnat , <&c. By Dr . Woodward. 255 
the number of rows of beads, varying with the angle of the illumi- 
nating pencil. Or the true appearance may be produced by the 
achromatic condenser adjusted so that the light is either truly 
central, or slightly oblique in the direction of the length of the 
scale ; and then a very moderate degree of obliquity in the illumina- 
tion transversely to the scale, obtained by means of the right-angled 
screws of the condenser, will bring out the rows of beads, varying 
in number as in the former case, in accordance with the degree of 
obliquity attained. 
I submit these results without further comment at the present 
time, with a few additional photographs intended to represent some 
of the chief appearances obtained. Two of these pictures, in addi- 
tion to that mentioned above, are from the slide of gnat scales sent 
by Mr. Mayall, and are taken with the immersion T Vth of Powell 
and Lealand ; the others represent a mosquito scale as seen with an 
immersion T Vth, constructed for the Museum by Mr. Tolies, of 
Boston. I selected for this series a different lens from that used 
for the gnat scales to show that the diffraction appearances dis- 
cussed in this paper result from the optical conditions under which 
the scales are viewed, and not from any peculiarity in the objectives 
of any particular maker. 
In conclusion, I would refer those who desire preliminary 
information as to the character and distribution of the gnat scales 
to the paper by Mr. Jabez Hogg, “ On Gnat Scales,” in this 
Journal for October, 1871, p. 192 ; or to his work on the Micro- 
scope, the first edition of which was published in 1854. The 
description there given of the various forms of gnat scales, and of 
their distribution on the insect, is very nearly accurate for the 
mosquito also. 
List of Photographs. 
A. Gnat scale, showing three rows of intercostal beads. Magnified 1350 dia- 
meters by Powell and Lealand’s immersion T ^th. (Neg. 771.) See PI. 
CXXXIX., Fig. A. 
A smaller gnat scale, showing smooth transverse markings. Magnified 
1500 diameters ; same objective. Achromatic condenser ; central light. 
(Neg. 7S1.) See id., Fig. B. 
0.— The same scale, with same objective and power, but moderate obliquity of 
illumination obtained by means of the right-angled screws of the con- 
denser. (Neg. 782.) See id., Fig. C. 
D. — Mosquito scale, showing smooth transverse markings. Magnified 1350 dia- 
meters by an immersion T qth of Tolies. Achromatic condenser ; nearly 
central light. (Neg. 765.) See PI. CXL., Fig. D. 
E. Same scale, same objective, but light oblique laterally as well as trans- 
versely, showing two rows of beads in each intercostal space. 1350 dia- 
meters. (Neg. 768.) See id., Fig. E. 
F. — Same scale, &c., showing three rows of beads in each intercostal space. 
1300 diameters. (Neg. 778.) See kl.. Fig. F. 
G. — Same scale, &c., showing four rows of beads in each intercostal space. 
1350 diameters. (Nog. 766.) See id.. Fig. G. 
