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Dr. Royston-Pigott on an Aplanatic Searcher. [Apr. 28, 



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 emanating 

 from a brilliant microscopic origin of light. 



It then appeared that delusive images, which the writer has ventured to 

 name cidola*, exist in close proximity to the best focal point (where the 

 least circle of confusion finds its locus). 



(I.) That these images, possessing extraordinary characters, exist princi- 

 pally above or below the best focal point, 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 conveniently two 

 thousand times less than a known original) are formed (under precautions) 

 with a remarkable freedom from aberration, which appears to be reduced 

 in the miniature to a minimum. 



(IV.) The beauty or indistinctness with which they are displayed (espe- 

 cially on the immersion system) is a marvellous test of the correction of the 

 observing objective, but an indifferent one of the image-forming objective 

 used to produce the testing miniature. 



These results enable the observer to compare the known with the un- 

 known. By observing a variety of brilliant images of known objects, as 

 gauze, lace, an ivory thermometer, and sparkles of mercury, all formed in 

 the focus of the objective to be tested with the microscope properly ad- 

 justed so that the axes of the two objectives may be coincident, and their 

 corrections suitably manipulated, it is practicable to compare known delu- 

 sions 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. 



A. A lengthened tube increased aberration faster than it gained power 

 (roughly the aberration varied as v 2 , while the power varied as v), 



B. As the image was formed by the objective at points nearer to it than 

 the standard distance of nine inches, for which the best English glasses 

 are corrected, the writer found the aberration diminished faster than the 

 power was lost, by shortening the body of the instrument. 



C. The aberration became negatively affected, and required a positive 

 compensation. 



D. Frequent consideration of the equations for aplanatism suggested the 



* From eiPwXov, a false spectral image. 



