ON INSTRUMENTS AND THEIR USE. 13 



delicate markings on the siliceous valves of Diatoms, 

 or the artificial ruled lines on glass (Nobert's test) 

 require to be made out, that powers beyond 1,200 

 diameters become useful ; and it must be admitted 

 that the scientific investigator does not lose any- 

 thing, nor are any of his philosophical deductions 

 vitiated by eschewing such powerful instruments, 

 which, in the hands of the skilful, succeed in 

 amplifying and resolving certain pretty structures ; 

 but, to the ordinary worker are, in reality, impedi- 

 ments to research. 



On the threshold of this inquiry, it thus becomes 

 evident, that an acquaintance with the general 

 structure of the human eye, coupled with the 

 principles of luminous energy, are necessary adjuncts 

 to an understanding of the microscope, its use and 

 power. 



The Human Eye. 



DESCRIPTION OF PLATE. 



&, Anterior chamber of eye filled with the 

 aqueous humour. 



c, Cornea, i, iris, p, pupil. 



cm, Ciliary muscle, cp, ciliary processes. Z, lens. 



s, Sclerotic coat, c, choroid. r, retina. 



/, Fovea. RR, superior and inferior recti muscles. 



0, Optic nerve entering the sclerotic and choroid 

 coats. 



v, Vitreous humour. 



In all studies, whether of pure Microscopy as a 

 Science, or whether of one of those departments of 

 natural history in which the microscope is employed 



