158 A New Formula for a Microscope Object-glass. 



inconsistent with theory, a brief retrospect of the early improve- 

 ments of the microscope object-glass will help to define the condi- 

 tions. The knowledge of its construction has been entirely in the 

 hands of working opticians ; and the information published on the 

 subject being scanty, this has probably prevented the scientific 

 analyst from giving that aid which might have been expected. 



Previous to the year 1829 a few microscopic object-glasses were 

 made, composed of three superposed achromatic lenses ; but this 

 combination appears to have been used merely with the intention of 

 gaining an increase of power, in ignorance of any principle, and 

 without even a knowledge of the value of angular aperture. 



At this time the late J. J. Lister tried a number of experiments, 

 and discovered the law of the aplanatic focus, and proved that, by 

 separating lenses suitably corrected, there were one or two positions 

 in which the spherical aberration was balanced. This was explained 

 in a paper read before the Eoyal Society in 1829. In the year 

 1831 Mr. Eoss was employed to construct the first achromatic 

 object-glass in accordance with this principle, which performed with 

 " a degree of success never anticipated." 



Mr. Eoss then discovered that, after he had adjusted the interval 

 of his lenses for the aplanatic focus, that position would no longer 

 be correct if a plate of thin glass was placed above the object ; this 

 focus had then to be sought in a different plane, and the lenses 

 brought closer together, in order to neutralize the negative aberra- 

 tion caused by covering-glass of various thickness. From this 

 period the " adjustment " with which all our best object-glasses are 

 now provided became established. Fig. 1 is the form of object-glass 

 used at this time, consisting of three plano-concave achromatics, 

 whose foci were nearly in the proportion of 1, 2, 3. 



No greater angle than 60° could be obtained with this system 

 in a ^-inch objective (the highest power then made), for reasons 

 apparent in the diagram. The excessive depth of curvature of the 

 contact-surfaces of the front pair is unfavourable for the passage of 

 the marginal rays ; the softness of the flint glass forming the first 

 plane was also objectionable. In the year 1837 Mr. Lister gave 

 Mr. Eoss a diagram for an improved " eighth," having a triple 

 front lens in the form shown in Fig. 2. By this the passage of 

 extreme rays was facilitated ; and in order to diminish the depth of 

 curvature, a very dense glass was used, having a specific gravity of 

 4 351. Faraday's glass, having a density of 6*4, had been pre- 

 viously tried, but was abandoned on account of a difficulty in 

 working it. The polished surfaces of both these qualities of dense 

 glass speedily became tarnished by exposure to the air ; and thus 

 the dense flint concave could only be employed in a triple combina- 

 tion, that is, when cemented between two lenses of crown glass : 

 this form of front was kept a trade secret, and was not published in 



