MAGNIFYING POWERS. 171 



SO completely were the errors of sphericity and dispersion 

 balanced or destroyed — that the circumstance of covering the 

 object with a plate of the thinnest glass or mica disturbed the 

 corrections, if they had been adapted to an uncovered object, 

 and rendered an object-glass, which was perfect under one 

 condition, sensibly defective under the other. This defect, 

 if that should be called a defect which arose out of improve- 

 ment, was first discovered by Mr. Eoss, who immediately 

 suggested the means of correcting it, and presented to the 

 Society of Arts, in 1837, a paper on the subject, which was 

 published in the fifty-first volume of their Transactions, and 

 which, as it isy like Mr. Lister's, essential to a full under- 

 standing of the ultimate refinements of the instrument, we 

 shall extract nearly in fuU : — 



" In the course of a practical investigation," says Mr. Ross, 

 " with a view of constructing a combination of lenses for the 

 object-glass of a compound microscope, which should be free 

 from the effects of aberration, both for central and oblique 

 pencils of great angle, I combined the condition of the greatest 

 possible distance between the object and object-glass ; for La 

 object-glasses of short focal length, their closeness to the 

 object has been an obstacle in many cases to the use of high 

 magnifying powers, and is a constant source of inconve- 

 nience. 



" In the improved combination, the diameter is only suffi- 

 cient to admit the proper pencil; the convex lenses are 

 wrought to an edge, and the concave have only sufficient 

 thickness to support their figure ; consequently, the combina- 

 tion is the thinnest possible ; and it follows that there will be 

 the greatest distance between the object and the object-glass. 

 The focal length is one-eighth of an inch, having an angular 

 aperture of 60°, with a distance of one-twenty-fifth of an 

 inch and a magnifying power of 970 times linear, with perfect 

 definition on the most difficult Podura scales. I have made 

 object-glasses of one-sixteenth of an inch focal length ; but as 

 the angular aperture cannot be advantageously increased, if 

 the greatest distance between the object and the object-glass 

 is preserved, their use will be very limited. The quaUty of 



