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X. — On the Existence of A car i between the Lamince of Mica in Optical Contact. 

 By Sir David Brewster, K.H, D.C.L., F.R.S. (Plate VI.) 



(Read 28th April 1862.) 



In searching for crystals of titanium between films of mica, I was surprised 

 to observe something resembling the remains of insects. Upon submitting the 

 specimen which contained them to a more careful examination, I found, in dif- 

 ferent parts of it, five completely formed Acari, all of which were much smaller 

 than those found in cheese, brown sugar, and damp bran, specimens of which 

 I obtained for the purpose of comparison. 



The plate of mica, now on the table, which contains these five Acari, is about 

 05 inches long, 3 inches broad, and nearly the ^th of an inch thick. The dis- 

 tances of the Acari from the edges of the plate are 1-6 inch, 1-13, 04, I'O, and 

 0-5. The largest of the Acari is about the ^th of an inch long, and the smallest 

 the ^th of an inch. 



The films of mica between which the Acari lie were in optical contact— that 

 is, no light was reflected from their contiguous surfaces. This fact does not prove 

 that the surfaces had never been separated ; for we may partially lift one film of 

 mica from another, so that light is reflected from the separated surfaces, and 

 yet they will return into optical contact. This is true even in a more rigid sub- 

 stance, such as glass, where a crack or fissure, produced in a thick plate by heat, 

 will reflect light at the separated surfaces, and will afterwards entirely disappear, 

 sometimes after the lapse of a few seconds, and sometimes after a much longer 

 interval, according as the surfaces have been more or less separated.* 



That these Acari were introduced between the laminae from without, cannot 

 be a matter of doubt ; but in what manner, and in what stage of their existence, 

 I do not presume to conjecture. 



When I first discovered these Acari, they were much more perfect than they 

 are in the accompanying drawings — executed for me by the Hon. Mrs Ward — as 

 will appear from the rude and highly magnified sketches of the largest, made by 

 myself They had obviously suffered compression during their carriage by post, 

 or perhaps from the object-glass of the microscope having been accidentally forced 

 upon the plate in the process of searching for them with high powers. 



If the specimen of mica is thought of sufficient interest to be preserved in the 

 Museum of the Society, with the drawings that are not engraved, it may be proper 

 to mention, that in trying to obtain a better view of the Acarus in the centre of 

 the plate, Mrs Ward detached a film, which she afterwards replaced with cement. 

 This fact is mentioned on her rough sketch of the plate, lest it might be supposed 

 that the Acarus at that place had been artificially introduced. 



* See Philosophical Transactions, 1816, pp. 73, 74. 

 VOL. XXIII. PART I. 2d 



