PRIMITIVE FORM OF LENS CORRECTION 



357 



English makers, and undoubtedly carried the palm both here and on 

 the Continent for the excellence of his objectives. 



1 inch 14 two doublets, 1832. Made for Mr. R. H. Solly. 



18 single triple, 1833. 



55 three pairs, 1834. This belonged to Professor Quekett. 



63 o 



44' 

 63 



74 



1 triple 



front and two double backs 



'}, Lister's formula 



1842. 



FIG. 310. A J-in. 

 combination by 

 Andrew Ross. 



Examples of these old lenses are extant and in perfect preserva- 

 tion, and for correction they are comparable without detriment to 

 any ordinary crown and flint glass achromatic of the same aperture 

 of the present day. 



An example of the construction of the J-inch focus objective of 55, 

 consisting of three pairs of lenses arranged with their plane sides 

 to the object, the position of least aberration, is shown 

 in fig. 310. The foci of these three pairs are in the 

 proportion of 1 : 2 : 3. In 1837 this maker had so 

 completely corrected the errors of spherical and 

 chromatic aberration that the circumstance of cover- 

 ing an object with a plate of the thinnest glass was 

 found to disturb the corrections ; that is to say, the 

 corrections were so relatively perfect that if the 

 combination were adapted to an uncovered object, 

 covering the object with the thinnest glass intro- 

 duced refractive disturbances that destroyed the high quality of the 

 objective. 1 



Lister's paper of 1830 gave the obvious clue to a method of 

 neutralising this ; that is to say, by lens distance ; and Ross applied 

 this correction by mounting the front lens of 

 an objective in a tube which slid over another 

 tube carrying the t\vo other pairs. A very 

 primitive form of this lens correction is afforded 

 us by a J-inch objective made by Andrew Ross 

 in 1838. It belonged originally to Professor 

 Lindley, the second President of the Royal 

 Microscopical Society, and was presented to the 

 society by his son, the Master of the Rolls, in 

 1899. An illustration of this lens is given in 

 fig. 311. The tube carrying the front lens 

 slides on an inner tube ; it can be clamped in 

 any position by the screws at the sides ; the 

 line in the small hole in the front indicates its 

 position, and is the prototype of the ' covered ' 

 and ' uncovered ' lines of later times. 



The larger cylinder at the base is the lid of its box upon which 

 it is standing. 



Subsequently this arrangement was modified by the introduction 



i Vide Chapter I. 



FIG. 811. Primitive 

 form of lens correc- 

 tion (1838). 



