INSECTS AND ARACHNIDA. 227 



than any photographic representation previously obtained; and it is clear 

 that Dr. Woodward regards this as the truest view. " Immediately after- 

 wards," he says, "with the same optical combination and magnifying 

 power, without any change in the cover-correction, by simply rendering 

 the illuminating pencil oblique, and slightly withdrawing the objective 

 from its first focal position, I obtained a negative which displays the 

 ' bead-like ' or varicose appearance of the ribbing more satisfactorily than 

 I had previously been able to do." 1 The beaded appearance shown in 

 this photograph, a copy of a portion of which is given in Fig. 421, cor- 

 responds so entirely with that which Dr. Woodward afterwards found to 

 be producible in the scale of the Gnat by a like alteration in the illumina- 

 tion ( 156), that the Author feels fully justified in adhering to his ori- 

 ginal opinion that it does not represent real structure, but is an optical 

 effect of diffraction. 2 



624. The Hairs of many Insects, and still more of their larvae, are 

 very interesting objects for the microscope, on account of their branched 

 or tufted conformation: this being particularly remarkable in those 



FIG. 421. 



Portion of a Podura-scale, from a Photograph by Col. Dr. Woodward. 



with which the common hairy Caterpillars are so abundantly beset. 

 Some of these afford very good tests for the perfect correction of Objec- 

 tives. Thus the hair of the Bee is pretty sure to exhibit strong prismatic 

 colors, if the Chromatic aberration should not have been exactly neutral- 

 ized; and that of the larva of a Dermestes (commonly but erroneously 

 termed the ( bacon-beetle ') was once thought a very good test of defining 

 power, and is still useful for this purpose. It has a cylindrical shaft 

 (Fig. 422, B) with closely-set whorls of spiny protuberances, four or five 



1 "Monthly MicroscopicalJournal," Vol. v., p. 246. 



2 The successive Volumes of the "Monthly Microscopical Journal," from the 

 2d (in which Dr. Royston-Piggott's views were first promulgated) to the present 

 date, teem with Papers on this subject from Mr. Jos. Beck, Mr. Mclntire, Dr. 

 Maddox, Dr. Royston-Pigott, Mr. Wenham, and Col. Dr. Woodward; which, 

 with a Paper by Mr. Slack in " The Student," Vol. v., p. 49, and a Paper by Mr. 

 Morehouse, giving the results of his examination of the scales of Lepisma and 

 Podura as opaque objects, under very high immersion objectives, with Beck's 

 Vertical Illuminator ("Monthly Microsc. Journ.," Vol. xviii., 1877, p. 31), should 

 be consulted by such as wish to follow out the inquiry. 



