130 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Highest Magnifying Powers.* — Mr. A. Y. Moore also refers to 



the same subject under this title. 



It is well known to all practical microscopists that the magnifying 

 power of an objective may be increased by eye-piecing to a certain 

 extent, with a continued gain in resolving power. When the limit 

 of resolving power is reached the magnifying power may be further 

 increased, but nothing is gained, except in the apparent size of details 

 already shown. After this comes a period in which the magnifying 

 power may be increased almost indefinitely ; but it is now very 

 noticeable that the resolving power is impaired. The aberrations of 

 the objective interfere greatly with the image. In fact, it is here that 

 a lens is frequently said to " break down." 



These three stages may be conveniently studied in an ordinary 

 cheap ^ of 100°. With an amplification of 300 diameters such a lens 

 should easily resolve P. angulatiim, but try as best we can, the lines of 

 Surirella gemma will fail to be seen. Now, if a higher eye-piece be 

 applied, giving a power of 500 diameters, this diatom may be resolved. 

 Supposing this to indicate the limit of resolvability of the object, a 

 still higher eye-piece may be used ; but the resolution is simply shown 

 larger. This period probably will extend to 1000 diameters, but if 

 increased much beyond this less is seen at each increase of power. 



The extent to which these three stages may be carried is, of course, 

 dependent upon the quality of the objective and its angular aperture. In 

 testing objectives the magnifying power should be carried to the second 

 stage, for a lens is frequently defeated simply because the visual 

 angles subtended by the lines (or dots) are insufficient for recognition 

 by the eye. 



In a recent article in the ' American Journal of Microscopy ' a 

 magnifying power of 100,000 diameters is mentioned, obtained by 

 means of a Wales' yL. From the fact that P. angulatum was the 

 extent of its resolving power, it is seen that the lens was far into the 

 third stage of its magnifying power. Any such increase of power is, 

 so far as practical work is concerned, useless ; but the second stage is 

 what we need and want. Frequently details are seen, but are so small 

 as to tire the eyes ; while if enlarged by a higher eye-piece fatigue is 

 prevented. 



Mr. Moore suggests the question, What is the highest power ever 

 attained and used without losing resolving power, and what objectives 

 are best suited to yield such powers ? Will a -^^ or -^\, with 

 lower eye-piecing, give better results than a ^ or ^L with high 

 eye-pieces and the magnifying powers the same ? He is only able 

 from personal experience to give the result of using a -^^ eye-piece, 

 with a i objective of " 180° " (or 100° " balsam angle "), giving a 

 magnifying power of 32,500 diameters. With this he was able to see 

 the last three diatoms of the balsam Moller Platte clearly resolved. 

 The lines of No. 20 did not look exactly like " the pickets on a fence," 

 but more like a lean horse's ribs. The eye-piece was not certainly 

 easy to use, and sunlight was necessaiy to see anything at all. 



* Amor. .Journ. Micr , v. (1880) pp. 174-5. 



