CONSTRUCTION OP THE MICROSCOPE. 31 



field, his combination has not been excelled by any subsequent set of 

 three lenses. By Mr. Lister's combination he was enabled to produce 

 lenses which transmitted a pencil of fifty degrees with a large field cor- 

 rect in every part. la the paper referred to above he enters into many 

 interesting particulars, which, however, are not necessary to the com- 

 prehension of our subject. Mr. Ross presented to the Society of Arts, 

 in 1837, a paper on the subject, which was published in the 51st volume 

 of their Transactions. This being essential to a full understanding of 

 the ultimate refinements of the instrument, we give it in full : 



"In the course of a practical investigation, with the view of con- 

 structing a combination of lenses for the object-glass of a compound 

 microscope which should be free from the eifects of aberration, both 

 for central and oblique pencils of great angle, I obtained 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 magnifying powers, and is 

 a constant source of inconvenience. 



" In the improved combination the diameter is only sufficient 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 combination 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 -1^ of an inch, having an angular aper- 

 ture of 60°, with a distance of ^ 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 an inch focal length; but as 

 the angular aperture cannot be advantageously increased if the greatest 

 distance between the object and object-glass is preserved, their use will 

 be very limited. 



"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 balanced, together with the general 

 perfection of the workmanship. Now in WoUaston's doublets and 

 Holland's triplets there are no means of producing a balance of the 

 aberrations, as they are composed of convex lenses only; therefore the 

 best thing that can be done is to make the aberrations a minimum. The 

 remaining positive aberration in these forms produces its peculiar efiect 

 upon objects (particularly the detail of the thin transparent class), whiuu 

 may lead to misapprehension of their true structure; but with the 

 achromatic object-glass, where the aberrations are coi-rectly balanced, 

 the most minute parts of an object are accurately displayed, so that a 



