THE MICROSCOPE. 73 



elements less than ^ T of an inch, these elements cannot be seen by 

 the eye. If a house, or barn, or an elephant, is composed of struc- 

 tural factors no larger than something smaller than the -^l T of an 

 inch, these elements cannot be seen by the eye, any better than they 

 can in any object a thousand times smaller, and the reason is that the 

 angular aperture of the eye is not wide enough to permit these small 

 things to be seen. If a microscopical house, composed of elements 

 -g-iy of an inch in size could be amplified by lenses which did not have 

 an increased power of delineating small objects, it would make no 

 difference how much the apparent size of the house was amplified, 

 even up to 5,000 diameters, the structural elements of the house 

 would yet be invisible. 



Now in looking at a diatom which has lines so close together 

 that the lines and spaces between them cannot be delineated or re- 

 solved by the objective which is being used on account of its narrow 

 angular aperture, no amount of amplification of the image of the 

 diatom, by the objective and eye pieces, even up to 5,000 diameters, 

 will make the lines on the diatom visible. But now if an objective, 

 of greater or less magnifying power, be substituted, which has a suf- 

 ficient angular aperture to delineate these lines and spaces, they 

 can be seen, if the whole diatom is magnified only a few hundred 

 diameters, instead of 5,000. 



Magnifying or refractive power, then, or focal distance is not 

 the only factor necessary in an objective, though it was formerly 

 thought to be. The popular idea is even at the present time, that 

 the power of the microscope has direct relations only to the 

 magnifying power of the lenses, especially to the objective. Objec- 

 tives were formerly constructed with narrow angular apertures, even 

 when made with the highest amplifying power. The fact that an 

 objective was a y 1 ^, or a fa, was considered to be proof of its high 

 power. This is only proof that it can amplify the image of a diatom 

 a certain number of diameters, but is not proof that it can show the 

 finer lines of a diatom, or other equally small elements in a general 

 image which is magnified. Beal and Carpenter used objectives of fa 

 and even -fa inch focal distance, but with narrow angular aperture, 

 and the fact has been since learned that objectives of £, or T 1 i; inch 

 focal distance are far better instruments, providing they have wide 

 angular apertures. Perhaps the best objective made and which 

 reaches the limit of possibilities in the power of objec- 



