OPTICAL IMPROVEMENTS. 73 



of the objective to show the peripheral or marginal por- 

 tions of the field with the same sharpness as the central. 1 



A considerable accession of available power has been 

 effected by both Foreign and English manufacturers since 

 the appearance of the third edition of our book, 1859. In 

 those by continental makers, this has been achieved by 

 the intervention of a third medium between the external 

 surface of the objective, and the covering glass of the 

 object. We may particularize those of M. Hartnack 

 Oberhauser's successor, as being most conspicuous ; but 

 the objective, which is said to be superior to all yet manu- 

 factured, is the -g^th of an inch of Powell and Lealand, 

 who, some time before, produced good objectives of ^g-th 

 and ^ th o f an i nc ;h focus. In these object-glasses, exces- 

 sive angular aperture has been judiciously sacrificed to a 

 more comprehensive and practical utility. It should be 

 stated, however, that the priority of construction of an 

 effective ^th objective is due to Mr. Wenham, to whom 

 the microscope was largely indebted before he made this 

 glass in 1856. It was constructed on a principle differing 

 from that usually adopted in the high powers, in having a 

 single front lens. A single anterior lens transmits more 

 light, gives clearer definition, with any required extent of 

 aperture; from its simplicity, it is comparatively freer 

 from errors of workmanship, and the chromatic and 

 spherical aberrations can be more perfectly corrected. 

 Mr. Wenham says, "that such object-glasses will chal- 

 lenge comparison with the best made and more expensive 

 forms." 



Dr. Beale's account of the performance of the ^jth is, 

 that it is even better than the ^th inch. Plenty of light 

 for illuminating the objects to be examined is obtained by 

 the use of a condenser provided with a thin cap, having an 

 opening of not more than ^j^h inch in diameter. The 

 objects should be covered with the thinnest glass, or with 

 mica, and then there is room to focus to the lower surface 

 of very thin specimens, which can alone be examined by 

 such high powers. Particular attention is drawn to these 

 very high powers, because the statements recently made in 



(1) See an excellent paper by J. Plumer, Esq. " On the Choice of a Micro- 

 scope." Micros. Journ. vol. iii. p. 153. 



