MAGNIFYING POWER OF SHORT SPACES. 7 



structed on the same principles are yet totally different as to 

 their uses. The healthy visual orjjan, itself a perfect instru- 

 ment, " converses with its objects" at almost all distances, 

 and assists the other senses in becomings acquainted with the 

 form, position, and magnitude of material substances. The 

 microscope, on the other hand, all but a perfect instrument, 

 enables us to see clearly and to examine certain objects, 

 which from their small size and without its aid would be 

 indistinct, if not altogether invisible. It is restricted to the 

 small size and the short distance of its objects, and from its 

 very consti'uction it has magnifying powers which the eye 

 neither possesses nor requires. If the eye were endowed 

 with these, therefore, to the exclusion of its self-adjusting 

 properties, whereby it discerns common objects in the ordinary 

 way at great and small distances, it would be rendered com- 

 paratively useless as a visual organ. 



Hence it were folly to attempt to invest this organ with 

 functions, the possession of which would subject its owner to 

 the greatest inconvenience. An exemplification of this position 

 occurs to me in the case of short-sighted persons. 



When therefore we find ourselves enabled by a carefully- 

 devised experiment to detect, with the naked eye, certain 

 configurations upon or within an object which, we may sup- 

 pose, has never before yielded an image at all excepting 

 through the medium of a lens, we are not to imagine that we 

 are thereby infringing on the domains of the microscope, 

 which being constructed for this very purpose would present 

 us, perhaps, with an image ten thousand times as large and 

 distinct. But putting this instrument altogether out of con- 

 sideration, and throwing aside all extraneous assistance, we 

 are the rather to consider how the eye, which has certain 

 limits to distinct vision for short distances, can yet adjust 

 itself for spaces still smaller, and in so doing become con- 

 verted into a kind of natural magnifying glass. 



We have now therefore to turn our attention to certain 

 microscopic objects, which are to be examined and resolved 

 without a lens of any description ; and we are stiiuulated to 

 an investigation of this kind by recollecting what has been 

 already attained with respect to the magnitude of the images 

 of small apertures themselves, when placed undei' circum- 

 stances the most favourable for their inspection. Amongst 

 these we cannot fail to have noticed at least two conditions 

 necessary to be fulfilled in such investigations, viz. — First, 

 that the object be held very near to the eye ; and secondly, 

 that every ray of light, excepting what is required to illumi- 

 nate the object, be carefully excluded. The first insures an 



