TEST-OBJECTS. 175 



ment being of comparatively little importance, is that which is 

 most valued in the Cart-horse ; but for the ordinary Carriage -horse 

 or Roadster, the highest merit lies in such a combination of speed 

 and power with endurance, as cannot co-exist with the greatest 

 perfection in either of the two first. — The Author feels it the more 

 important that he should express himself clearly and strongly on 

 this subject, as there is a great tendency at present, both among 

 amateur Microscopists and among Opticians, to look at the attain- 

 ment of that Resolving power which is given by Angular aperture 

 as the one thing needful, those other attributes which are of far 

 more importance in almost every kind of Scientific investigation 

 being comparatively little thought of. It is neither the only nor 

 yet the chief work of the Microscope (as some appear to suppose) 

 to resolve the markings of the siliceous valves of the Dialomacece ; 

 in fact the interest which attaches to observations of this class 

 per se is of an extremely limited range. If one-tenth of the 

 attention which these objects have received, had been devoted to 

 the careful study of the Life-history of the tribe of Plants which 

 furnishes them, it cannot be doubted that great benefit would have 

 accrued to Physiological Science.* And the more carefully we look 

 into the history of those contributions to our knowledge which 

 have done most to establish the value of the Microscope as an in- 

 strument of scientific research, the more clear does it become that 

 for almost every purpose except the resolution of the Diatom-tests, 

 Objectives of moderate Angular Aperture are to be decidedly 

 preferred. 



132. Test- Objects. —It is usual to judge of the optical perfection 

 of a Microscope by its capacity for exhibiting certain objects, 

 which are regarded as Tests of the merits of its Object-glasses ; 

 these tests being of various degrees of difficulty, and that being 

 accounted the best instrument which shows the most difficult of 

 such tests. Now it must be borne in mind that only two out of 

 the four qualities which have been just enumerated — namely, 

 Defining power and Resolving power— can be estimated by any of 

 these tests ; and the greater number of them, being objects'whose 

 surface is marked by lines, striae, or dots, are tests of Resolving 

 power, and thus of Angular Aperture only. Hence, as already 

 shown, an Objective may show very difficult test-objects, and yet 

 may be very unfit for ordinary use. Moreover, these Test-objects 

 are only suitable to Object-glasses of very short focus and high 

 magnifying power ; whereas the greater part of the real work of 

 the Microscope is done with Objectives of low and medium power ; 

 and the enlargement of the Angular Aperture, which enables even 

 these to resolve (under deep Eye-pieces) many objects which were 



* The discovery of the conjugation of the Diatomacese (Fig. 122) by 

 Mr. Thwaites was made by means of an instrument certainly not 

 superior to the " Society of Arts Educational Microscope " (Fig. 31). 



