XVI 



INTRODUCTION. 



ward and forward motion of an object, the lateral motion being- effected by direct applica- 

 tion of the fingers. If the body of the microscopa is to be used in the inclined position 

 the sliding piece or a moveable stage becomes essential. 



If the moveable stage be present, the " milled heads " should be pretty large, so as to 

 be readily grasped, and a flat object should remain in focus whilst traversing the field by 

 the movement of the stage. The stage should also be very thin. 



The mirror should have one plane or flat face, and another concave. It should not be 

 too small ; and its centre should coincide with the axis of the body of the microscope. A 

 double arm enables the mirror to be brought more considerably to either side, so as to throw 

 more oblique light upon an object. 



So long as the above conditions are f ulfilled, the general form and arrangement of the stand 

 and its parts are of little consequence. It must also be remembered that the complication and 

 accuracy of the apparatus required will vary according to the kind of investigations pursued- 

 Thus the structure of the various tissues of animals, and that of most plants, can be satisfac- 

 torily studied with apparatus which is totally insufficient to display the structure of certain of 

 the more minute and difficult objects; but, on the other hand, it follows that if a peculiar 

 structure can be shown to exist in any hind of objects by a complicated apparatus, which 

 cannot be demonstrated by a more simple or less perfect apparatus, the study of the struc- 

 ture of any obj ect 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 micro- 

 scopist 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, often called Objectives, — These form the most important portions of the 

 microscope. The value 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 or outlines of objects or structures ; whilst large angular aperture renders them 

 capable of penetration, or of resolving the more delicate markings upon the surface of 

 objects. But 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 the French and German object-glasses have been very greatly im- 

 proved in late years, and are now largely used, especially the immersion-glasses as they are 

 called (Object-Glasses) ; and some of these resolve perfectly most of the difficult valves 

 of the Diatomaceae ; and they are cheaper than the English glasses. The names of Ilart- 

 nack, Zeiss, and Guncllach are well known as those of excellent makers. Some of the 

 American object-glasses also, which are but little known in this country, must stand in the 

 first rank in regard to excellence in defining and especially penetrating power. When a 

 glass of unknown value, however, presents itself, it should be tried upon the test-objects. 



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

 figs. 1 to 4. 



The outlines or margins of these objects must appear black, well defined, and perfectly 

 free from colour, not misty, nor red or green ; they should retain this appearance when the 

 higher eyepieces are used, of course some allowance being made in regard to the sharpness 

 of outline, which will appear slightly broader and less defined, but nowise interfering 



