126 TRANSACTIONS OP THE 



which will unite with the covering glass, an increase of light and 

 definition is obtained, as the most divergent rays are by their 

 passage through water, instead of through air, so refracted that 

 they enter the object-glass. 



The object-glasses last described have been patented by Mr. 

 Wenham, and they are made by the celebrated firm of Ross & Co. 

 in London, at prices considerably below those of their great 

 rival object-glassmakers, Powell & Lealand, whose ^ and yb""^^- 

 immersion lenses have for a considerable time been regarded as 

 the most perfect glasses made. The latter firm has just brought 

 out a ^ and a ^ constructed on a new formula, which they say are 

 superior to their former glasses of the same focal length. These 

 new glasses cost, with immersion arrangements, eleven guineas 

 a-piece. 



The qualities a first-rate object-glass should possess are, 



1. Defining power, which depends upon the completeness of its 

 corrections. 



2. Penetrating power, or focal depth, which depends upon the 

 degree of distinctness with which part of the objects that are a 

 little out of focus may be seen. 



3. Resolving power, by which minute surface markings are 

 seen and clearly separated from each other. 



4. Flatness of field, which depends on the distinctness with 

 which both the central and peripheral portions of an object are seen. 



An object-glass combining all the preceding attributes in the 

 highest degree in which they are compatible with each other may 

 be pronounced perfect. 



The optical qualities of an object-glass are examined by means 

 of tests or test-objects, such as sections of spines of echinus, of 

 deal, proboscis of fly, pygideum of flea, muscular fibres, finely 

 marked valves of diatoms, scales of insects, especially the Podura 

 scales, and Nobert's test-plate having as many as 200,000 parallel 

 lines to the inch ruled on a piece of glass with a diamond. I 

 cannot refrain from mentioning here the marvellous Micro-engrav- 

 ings of Mr. W. Webb, whose finest engraving of the Lord's Prayer 

 is on a scale equal to that of writing the whole Bible fifty times 

 in the space of only one square inch.* 



* These engravings, Nobert's test-lines, Holler's typenplatten cand test- 

 objects, are supplied by Mr. Edmund Wheeler, 48 ToUington Eoad, 

 Holloway, London, N. 



