INVERTEBEATA, CRYPTOaAMIA, MICROSCOPY, ETC. 785 



lu its general scope the paper is intended to deal with difficulties 

 in microscopic research, usually found insuperable, such, for instance, 

 as the invisibility of minute closely packed refracting spherules, ex- 

 isting in double rouleaux, or promiscuously aggregated ; when their 

 individual diameter varies between the - ^^^^ to the ^ ^ ^^^ ^ ^ of an 

 inch. 



These difficulties are principally created by overlapping images, 

 due partly to residuary aberration both spherical and chromatic, 

 partly to the effects of diffraction, caused by brilliant illuminations 

 of spurious disks of light, partly to the constant development of 

 Eidola or false images, which vary the loci of their development 

 according to the nature of underlying structures, and according to 

 the object-glasses being over- or under-corrected, and partly, and 

 indeed very considerably, created by the use of excessively large 

 angular apertures. 



The pajjer discusses also the relative eff'ects on visibility, of large 

 and small angular apertures in objectives. 



It shows that the black margins or black marginal annuli of 

 refracting spherules, constantly displayed by low aperture glasses, 

 are attenuated gradually to invisibility, as the glasses employed are 

 endowed with the largest apertures ; that the black margins also of 

 cylinders, tubules, or semi-tubules suffer similar obliterations ; and 

 that, in consequence, innumerable minute details are concealed or 

 destroyed till the aperture is sufficiently reduced; that minute 

 refracting bodies obey the laws of their refrangibilities, and display 

 beautiful phenomena, discoverable by transcendent powers of defini- 

 tion, but totally unseen by inferior compensations ; and that, in con- 

 sequence, the so-called achromatism of modern glasses is an illusory 

 approximation to correct vision. 



Examples are given of molecular structures, varying in form, 

 translucency, and refrangibility, in which natural pencils arc caught 

 and displayed in the ordtr in which, as in a rain-drop, iridescent rays 

 are emitted by the decomposed light. Several examples are also 

 introduced, in which a high order of lenticular correction beautifully 

 discovers structure, hidden, according to Dr. Carpenter, from the 

 great bulk of observers. 



As the paper deals so often with magnitudes very much less 

 than the xooVo^ "^^ *^ inch, a method is introduced of readily esti- 

 mating roughly such magnitudes between the -g^^oo and the 500V00" 

 of an inch, by means of a micrometer gauge. The writer has been 

 emboldened to grapple with these difficult minutite, in consequence of 

 the sharp and clear definition he has attained of spider lines miniatured 

 down to the fourteenth part of jTyijoTro ^^ ^^ inch. The eye, ac- 

 customed to contemplate this subtlety of form, readily appreciates the 

 one-fourth or one-sixth of this size, i. e. 4o7iVo o' ^^ TrWorro- 



A new test for the Microscope is also described displaying bright 

 lines of uniform thickness less than y ou^ooo ? ^^^ sharp black lines of 

 much less tenuity than those given by Nobert's celebrated lines 

 ruled on glass, and incomparably more easy of illustration. 



The employment of various fluids for immersion lenses is care- 



