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 xVth of Powell 

 and Lealand ; the others represent a mosquito scale as seen with an 

 immersion tV^^^j constructed for the Museum by Mr. Tolles, of 

 Boston. I selected for this series a different leus 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 tliree rows of intercostal beads. Magnified 13.50 dia- 

 meters by Powell and Lealand's immersion ^'^th. (Neg. 771.) See PI. 

 CXXXIX., Fig. A. 



B. — A smaller gnat scale, siiowing smooth transverse markings. Magnified 

 1500 diameters ; same objective. Achromatic condenser ; central light. 

 (Neg. 781.) See id., Fig. B. 



C. — The same scale, with same objective and power, but moderate obliquity of 

 illumination obtained by means of the rigiit-angled screws of <he con- 

 denser. (Neg. 782.) See id.. Fig. C. 



D. — Mosquito scale, showing smooth transverse markings. Magnified 1350 dia- 

 meters by an immersion ^gth of Tolles. 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 id.. Fig. F. 



Ct. — Same scale, &c., showing four rows of beads in each intercostal space. 

 1350 diameters. (Neg. 766.) See id., Fig. G. 



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