92 Transactions of the 



Weubam, reprinted verbatim from the ' Proceedings of the Royal 

 Society,' on " A New Formula for a Microscope Object-glass." In 

 this combination the front lens is a single plano-convex of unusual 

 thickness, being, in ^fact, considerably thicker than a hemisphere ; 

 behind this is a plano-convex triple, the middle lens of which is a 

 double concave of dense flint-glass ; and behind this a plano-convex 

 single lens, the plane side of which is uppermost. The focal lengths 

 of these lenses are about in the ratio of 1 : 3 : 4 • 5. It may here be 

 remarked that the diagram Fig. 6, representing the halves of this 

 and a prior construction of Mr. Wenham's, is incorrectly shaded, 

 and therefore rather misleading ; Jiint and crown glass should be 

 respectively similarlij shaded on both sides of the diagram. In the 

 absence of any verification of the projection of the diagram, the 

 statement of Mr. Wenham respecting the paths of the extreme rays 

 through the combination may, without doubt, be taken for granted. 

 From this it appears that these rays are, on their final emergence 

 from the posterior surface, brought mto a state of parallel coinci- 

 dence, in place of converging near the conjugate focus, as was 

 generally the case with objectives of the old construction. An 

 important advantage of this appears to be that more distant conju- 

 gate foci may safely be employed ; or, in other words, that amplifi- 

 cation of the image by elongation of the body of the microscope may 

 be effected with less loss of definition than in objectives of the usual 

 type. The writer has long since satisfied himself that with the 

 objectives in his possession, comprising some considered to be first- 

 rate by their respective makers, there is less loss of both light and 

 definition in obtaining increased magnifying power by elongation of 

 the body of the microscope than by the employment of very deep 

 eye-pieces. Another advantage of the new formula appears to be 

 that when the glass is in adjustment for covered objects, sufficient 

 space remains for further approximation of the single front to the 

 triple lens in order that an adjustment for immersion may be eff'ected 

 Avithout any change of lenses. The writer has long desired an 

 opportunity of comparing some objectives made on the new formula 

 with those of similar power of the usual construction, but — perhaps 

 because the demand for these glasses has been in advance of the 

 supply, for it appears that the tact and delicacy of hand required 

 for the manufacture of the minute lenses of objectives of high power 

 is rather a question of intuition than of mere training — that wish 

 has not as yet been complied with.* 



* Since this statement was made, an opportunity has occurred of comparing 

 several of the new objectives, ranging from \ to -j'5 in. focus, with others of the 

 old model ; and it is unquestionable tliat they have no reason to fear comparison 

 with the very best objectives of the old pattern. The only exception (quantum 

 valcat) that can be taken to those objectives is that there is somewhat less flat- 

 ness of field ; but the writer cannot avoid expressing his conviction that in the 

 objectives of the future, and especially in the higher powers, the new formula 

 will supersede all those hitherto adopted. 



