394 



a search for the cause of defective definition, if any existed. 

 A variety of first-class objectives, from the V^ to the |, failed 

 to show the beadin^?, although most carefully constructed by 

 Messrs. Powell and Lealand. 



Experiments having been instituted on the nature of the 

 errors, it was found that the instrument required a better 

 distribution of power ; instead of depending upon the deepest 

 eyepieces and most powerful objectives hitherto constructed, 

 that better effects could be produced by regulating a more 

 gradual bending or refraction of the excentrical rays emanat- 

 ing from a brilliant microscopic origin of light. 



It then appeared that delusive images, Avhich the writer 

 has ventured to name eidola,^ exist in close proximity to the 

 best focal point (where the least circle of confusion finds its 

 locus). 



I. That these images, possessing extraordinary charac- 

 ters, exist principally above or below the best focal jDoint, 

 according as the objective spherical aberration is positive or 

 negative. 



II. That test-images may be formed- of a high order of 

 delicacy and accurate portraiture in miniature, by employing 

 an objective of twice the focal depth, or, rather, half the focal 

 length of the observing objective. 



III. That such test-images (which may be obtained con- 

 veniently two thousand times less than a known original) are 

 formed (under precautions) with a remarkable freedon from 

 aberration, which appears to be reduced in the miniature to 

 a minimum. 



IV. The beauty or indistinctness with which they are dis- 

 played (especially on the immersion system) is a marvellous 

 test of the correction of the observing objective, but an indif- 

 ferent one of the image-forming objective used to jiroduce 

 the testing miniature. 



These results enable the observer to compare the known with 

 the unknown. By observing a variety of brilliant images of 

 known objects, as gauze, lace, an ivory thermometer, and 

 sjiarkles of mercury, all formed in the focus of the objective 

 to be tested with the microscope properly adjusted so that 

 the axes of the two objectives may be coincident, and their 

 corrections suitably manipulated, it is practicable to compare 

 known delusions with suspected phenomena. 



It was then observed (by means of such appliances) that 

 the aberration developed by high-power eyepieces and a 

 lengthened tube followed a peculiar law. 



^ From lUoAov, a false spectral image. 



