GLASGOW SOCIETY OF FIELD NATURALISTS. 125 



two systems of lenses, the anterior, consisting of a plano-convex 

 front lens and a concavo-convex lens cemented together; the 

 posterior combination consists of a plano-concave and bi-convex, 

 (fig. 4) or of a bi-concave lens with a bi-convex lens above and 

 below (fig. 5). In higher powers these lenses become more 

 numerous, consisting of a triple anterior and posterior and a 

 double middle combination as shown in diagram No. 8. Mr. F. 

 H. Wenham has proved that a pencil of rays exceeding an angle 

 of 40° from a luminous point, cannot be secured with less than 

 three superposed lenses of increasing focus and diameter, but by 

 the use of such a triple combination rays beyond this angle can 

 be transmitted with successive reflections in their course towards 

 the posterior conjugate focus. The first object-glass made by 

 Mr. Ross in 1831, consisted of three bi-convex lenses of crown 

 glass, each of which was achromatized by its own plano-concave 

 lens of flint glass (fig. 6), It was in one of these glasses that 

 Mr. Koss made the anterior combination adjustable in order to 

 correct the errors of spherical aberration consequent upon using 

 covered or uncovered objects. In 1837, Mr. Lister gave Mr. Ross 

 a diagram for a -j-in., having a triple front lens to facilitate the 

 passages of the extreme rays. In 1850, the same gentleman 

 communicated to Mr, Wenham his invention of the triple back 

 combination (fig. 8), which was a considerable improvement on the 

 double one. Mr. Wenham, in the course of his trials, invented 

 the single front of crown glass, the front part being cylindrical 

 and the back convex. The height of the cylindrical part is one of 

 the most essential parts of corrections, as for instance, in a yL a 

 difierence of thickness of only -002 inch determines the quality 

 between a good and an indifierent glass. This single front devised 

 by Mr. Wenham has, after much hesitation on the part of other 

 makers, been adopted by them all, Mr. Wenham constructed an 

 excellent object-glass of five lenses only, and since then he has 

 devised object-glasses of only four lenses as shown in the diagram, 

 fig. 9, thus dispensing with six surfaces as formerly used. It is 

 obvious that these objectives, if properly corrected, must give an 

 increase of brilliancy and definition. Another advantage is, that 

 high powers constructed on this plan can be used both as dry and 

 as wet or immersion lenses, by simply adjusting the screw collar. 

 The system of immersion lenses is due to Professor Amici, who 

 discovered that by placing a small drop of water on the front lens 



