xiv INTRODUCTION. 



the lower forms of vegetable life, as the Diatomacese, &c. But, on the other hand, it follows, 

 that if a peculiar structure can be shown to exist in any kind of objects by a complicated 

 apparatus, which cannot be demonstrated by a more simple or less perfect apparatus, the 

 study of the structure of any object not previously examined, must always be attended with 

 uncertainty, so long as it has not been tested by the more perfect kind of apparatus ; 

 provided the microscopist has not acquired the art of replacing the imperfection of his 

 apparatus by superior tact and management, which can be done to a great extent. 



Object-glasses. The goodness of the object-glasses depends mainly upon their freedom 

 from chromatic and spherical aberration, and upon the magnitude of their angular aperture. 

 The freedom from the former renders them good in defining power, i. e. in exhibiting clearly 

 the margins of objects, whilst large angular aperture renders them capable of penetration, 

 or of rendering visible or distinct, markings upon the surface of objects. At least this is 

 the ordinary statement made in regard to the relations of defining and penetrating power ; 

 but it is only partially true, and there are two kinds of penetrating power, as we shall show 

 in the article "TEST OBJECTS," where we have entered more fully upon this subject. 



As in the case of the stand, &c. of microscopes, so in regard to the object-glasses, the 

 best are made in this country, and can be obtained of first-rate quality of the three makers 

 above mentioned. But when a glass of unknown value presents itself, it should be tried 

 upon the test objects. 



The defining power may be tested by the examination of the objects figured in Plate 1 . 

 figs. 1 to 4. 



The outlines of these objects must appear black, well defined and perfectly free from colour, 

 not misty and red or green ; they should retain this appearance when the higher eyepieces 

 are used, of course, some allowance being made in regard to this sharpness of outline, 

 which will appear slightly broader and less defined, but in nowise interfering with the 

 distinctness of the image of the object. The various parts of an object lying in the same 

 plane, as a transverse section of whalebone, should also be visible at the same focus ; the 

 lines upon a micrometer used as a slide will also serve to test this point. It is not, however, 

 of very great importance, especially with high powers ; but it is a character of a superior 

 object-glass. 



If the definition of the glass be good and the power adequately high, it will also exhibit 

 the structure of the objects in Plate 1. figs. 5, 6, 10, 12, and 13, clearly and distinctly; it is 

 then of sufficiently good quality for nearly all the purposes required in the investigation of 

 animal and vegetable structures. Some German and French glasses will do this tolerably 

 well (although many of those sold are worthless) ; but they are not provided with a correcting 

 adjustment to compensate for the effects of the varying thicknesses of the layer of liquid 

 and the glass cover through which objects are generally seen, so that the best working of 

 these glasses can only be obtained by accident. Still many of them are quite fit for all 

 ordinary investigations, so long as these are carried on in a proper manner. 



The exhibition of the objects illustrated by Plate 1. figs. 6, 7, 8, 9, 10, 11, 12, and 13, 

 requires the first kind of penetrating power, but it does not require large angular aperture. 

 The second kind of penetration, however, requires above all large angular aperture, 

 independently of any other superiority ; i. e. a glass maybe perfectly corrected as to defining 

 power, and exhibit the above objects well, yet when the valve of a Gyrosigma is subjected 

 to it, the markings cannot be distinguished without particular appliances, which produce 

 the same effect as an increase of angular aperture in the object-glass. As this property is 

 therefore principally dependent upon the angular aperture, this should be determined by 

 direct measurement; the method of doing which is described under the article "ANGULAR 



