98 Dr. Brewster on the Formation of Single Microscopes 



In Plate II. Fig. 12. we have given a sketch of the ap- 

 pearance of the vein of black lead, from the pencil of Mr. 

 Skene of Rubislaw, who has examined the mine, and to whom 

 we have been indebted for the preceding particulars. 



The letters B, B, B show the vein of black lead ore, and 

 C, C, C, the clay slate rock in which it occurs. 



Art. XVII. — On the Formation of Single Microscopes from 

 the Lenses of Fishes, #c. By David Bkewstee, LL.D. 

 F.R.S. and Sec. U.S. Edinburgh. 



Having been occupied for many years in a minute examina- 

 tion of the optical and anatomical structure of the lenses of 

 various animals, the idea has frequently occurred to me of 

 employing the lenses of the smaller ones as single microscopes. 

 In putting this idea to the test of experiment, however, I did 

 not at first obtain the results which I expected ; but this failure 

 arose principally from want of attention to several minute cir- 

 cumstances, which are essential to the success of the experi- 

 ment. 



In the examination of objects of natural history and ana- 

 tomy, cases frequently occur where the compound microscope 

 fails, and where a single lens can alone be advantageously 

 employed. Those who have been reduced to such a difficul- 

 ty, must have experienced the imperfections even of the 

 simple instrument, and must have abandoned inquiries which 

 promised to lead to new and important results. If the ob- 

 server has lenses ground by the first artists, and has even 

 taken the precaution of illuminating his objects with homoge- 

 neous light, so as to remove the indistinctness arising from the 

 different refrangibility of light, he has still to encounter the 

 equally formidable evil of spherical aberration, which in 

 small lenses at least, we fear we shall never be able to re- 

 medy. Having been often reduced to this dilemma, it ap- 

 peared to me not an unreasonable expectation, that when the 

 joint efforts of science and practical skill had failed, we might 

 have recourse to that pre-eminent wisdom, which He who 

 made the eye has displayed in the structure of the crystalline 

 lens ; and avail ourselves of those single microscopes which 



