324 PR0CEEDING9 OF SOCIETIES. [ 'jo" mal. JmmT?^a ' 



Lcaland's T%th immersion lens, representing exactly the appearances 

 made so familiar by the drawings of the late Richard Beck ; tho other 

 representing P. angulntum, and particularly remarkable for the round- 

 ness with which the dots are brought out. 



The following letter was then read to the meeting : — 



War Department, Surgeon-General's Office, 

 Mr. Jabez Hogg, Washington, D.C, April 23, 1870. 



Dear Sir, — I have been reading with great interest the papers of 

 Dr. Pigott on the nature of the markings of certain test-objects, and 

 particvdarly of the Podura scale. The novel views he advances struck 

 me as so interesting that I at once began to re-examine the specimens 

 I had on hand. I must confess that up to the present moment I have 

 been unable to satisfy myself of the correctness of his interpretation 

 of its phenomena ; and yet the fact that he appears to have convinced 

 so distinguished a microscoj)ist as the Eev. J. B. Eeade, makes me 

 incline to caution in expressing my opinion. 



I have had no difficulty in seeing the beaded appearance of the 

 Podura scale which he has described, " green upon a pink ground, or 

 pink upon a greenish ground,"* or greenish blue or blue on a ground 

 of various shades of red or the reverse, with various lenses, especially 

 the ith of Wales and the immersion -xt^i^ of Powell and Lealand, by 

 taking out the condenser and illuminating the scale by a parallel 

 pencil of sunlight reflected by a j)lane mirror placed somewhat 

 obliquely about 20 inches from the stage. I know, however, of no 

 method of illuminating the microscoiDC more likely to j^roduce inter- 

 ference phenomena, and should have set the appearance down as such 

 without hesitation but for the high authorities to which I have alluded. 

 As just intimated, the colour of the pseudo beads varied from green 

 to blue on the one hand, to pink or deep red on the other ; and this 

 appearance a2ii)eared to me to de})end simply on the degree of colour- 

 correction of the objective used, and corresponded to phenomena 

 observed with the same lens on other objects. Still I do not wish to 

 come to a premature conclusion, and write you with the hope that you 

 can obtain for me from Dr. Pigott two or three slides with scales in 

 which he has seen himself the appearance he has described. I should 

 like to make a careful examination of the matter, but desire to feel 

 sure that I am dealing with scales from the same species of insect, 

 since this has been made such a point. 



Since Mr. Pigott has politely alluded to certain photographs of the 

 Podura scale made under my sujjervision some time ago, I enclose 

 some made by myself since reading his paper, which show what I con- 

 ceive to be the true appearance of the scale. I should like very much 

 to sec a photograph representing Dr. Pigott's views in a manner satis- 

 factory to himself, but fear this is asking too much, unless indeed my 

 distinguished friend Dr. Maddox can be induced to undertake the 

 task. I also enclose a photograph I have just made of the Angulatum 

 scale, which exhibits still more distinctly the structure shown by the 



* ' Monthly Microscopical Journal,' December, 1869, p. 300. 



