66 PRACTICAL TREATISE ON THE MICROSCOPE. 



two. A glass of this form Is capable of transmitting as large 

 an angvilar pencil as 65° with perfect distinctness. 



The above described combination of three lenses approaches 

 so very closely to the objects to be examined, that they re- 

 quire to be covered with the very thinnest mica, which is 

 objectionable, and no more than three lenses can possibly, be 

 employed to form a single microscope ; hence the limit to the 

 improvement of this instrument. Mr. Holland states, that 

 for a triplet to be efficient for the podura, &c., it should be 

 equivalent in power to a single lens of one-twenty-fifth of an 

 inch focus; and in answer to those who object to the use of 

 the triplet, on account of its approaching so closely to the 

 object, he states that some of his preparations are covered 

 with mica so thin, that they can be examined by a spherule 

 of one-three -hundredth of an inch focus. " It was at one 

 time hoped," says Mr. Koss, " as the precious stones are more 

 refractive than glass, and as the increased refractive power is 

 unaccompanied by a corresponding increase in chromatic dis- 

 persion, that they would furnish valuable materials for lenses, 

 inasmuch as the refractions would be accomplished by shal- 

 lower curves, and, consequently, with diminished spherical 

 aberration."* But these hopes were disappointed: every- 

 thing that ingenuity and perseverance could accomphsh was 

 tried by Mr. Varley and Mr. Pritchard, under the patronage 

 of Dr. Goring., It appeared, however, that the great reflective 

 power, the doubly-refracting property, the colour, and the hete- 

 rogeneous structure of the jewels which were tried, much more 

 than counterbalanced the benefits arising from their greater re- 

 fractive power, and left no doubt of the superiority of skilfully 

 made glass doublets and triplets. The idea is now, in fact, aban- 

 doned ; and the same remark is applicable to the attempts at 

 constructing fluid lenses, and to the projects for giving to 

 glass other than spherical sin-faces, — ^none of which have come 

 into extensive use. 



* Art., Microscope, Penny Cychpisdia. 



