395 



A. A lengthened tube increased aberration faster than it 

 gained poAver (roughly the aberration varied as v^, 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 idea of searching the axis of the instrument for 

 aj)lanatic foci, and that many such foci would probably be 

 found to exist in proportion to the number of terms in the 

 equations (involving curvatures and positions). 



E. The law was then ascertained that power could be 

 raised, and definition intensified, by jjositively correcting the 

 searching lenses in proportion as they ajDproached the ob- 

 jective, at the same time applying a similar correction to the 

 observing objective. 



The chief results hitherto obtained may be thus sum- 

 marised. 



The writer measured the distance gained by the aplanatic 

 searcher, whilst observing with a half-inch objective with a 

 power of seven hundred diameters, and found it two tenths of 

 an inch increase ; so that optical penetration was attain- 

 able with this high power through plate-glass nearly one 

 quarter of an inch thick, whilst visual focal depth was j)ro- 

 portion ably increased. 



The aj^lanatic searcher increases the power of the micro- 

 scope from two and a half to five times the usual power ob- 

 tained with a third or C eyepiece of one inch focal length. 

 The eighth thus acquires the power of a twenty-fifth, the 

 penetration of a one fourth. And at the same time the lowest 

 possible eyepiece (3-inch focus) is substituted for the deep 

 eyepiece formed of minute lenses, and guarded Avith a 

 minutely perforated cap. The writer lately exhibited to 

 Messrs. Powell and Lealand a brilliant definition, under a 

 power of four thousand diameters, with their ncAV " eighth 

 immersion" lens, by means of the searcher and Ioav eyepiece. 



The traverse of the aplanatic searcher introduces remark- 

 able chromatic corrections displayed in the unexpected 

 colouring developed in microscopic test-objects.^ 



' Alluded to by Mr. Reade, F.K.S., iu the * Popular Science Review,' for 

 April, 1870. 



