30 THE MICKOSOOPE. 



circumstance of covering the object with a plate of the thin- 

 nest glass or talc 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 improvement, was first discovered by Mr. Boss, who imme- 

 diately 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 51st volume of their Transactions, and which, 

 as it is, like Mr. Lister's essential to a full understanding of the 

 ultimate refinements of the instrument, we shall extract nearly 

 in full: 



* ' In the course of a practical investigation (says Mr. Boss) 

 with the 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 pen- 

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

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

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

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

 ing powers, and is a constant source of inconvenience. 



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

 ficient to admit the proper pencil; the convex lenses are 

 wrought to an edge, and the concave have only sufficient thick- 

 ness to support their figure; consequently the combination is 

 the thinnest possible, and it follows that there will be the great- 

 est 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 l-25th 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 l-16th of 

 an inch focal length; but as the angular aperture cannot be ad- 

 vantageously increased, if the greatest distance between the 

 object and object-glass is preserved, their use will be very lim- 

 ited. 



"The quality of the definition produced by an achromatic 

 compound microscope will depend upon the accuracy with 

 which the aberrations, both chromatic and spherical, are bal- 



