xviii INTRODUCTION. 



ordinary glasses of 1-inch focus will show larger objects; hence enormous improvements 

 have latterly been made in object-glasses,— the increased magnifying power being produced 

 by the object-glasses and not by the eyepieces, by which means the visible images are 

 rendered most distinct and trustworthy ; for the object-glass alone produces the structural 

 image of the object, which is magnified by the eyepiece, but the latter elicits no further 

 structures. 



According to modern views, about 800 diameters is the magnifying power of the object- 

 glass which will resolve all visible structure. High eyepieces, even to 50,000 diameters, 

 will magnify the images, but will not elicit further structures. 



For ordinary useful purposes, the § or § inch and the J inch object-glasses, with the first 

 and third eyepieces, will sufiice. With the draw-tube and the third eyepiece, the J will 

 magnify 000 diameters, which is sufficient for most ordinary investigations. 



Recent investigators with very high powers, especially with immersion-lenses, have 

 brought to light new structures hitherto overlooked ; we need merely mention the muscle- 

 rods (Plate 22. fig. 36 c), the spermatozoal membrane (Plate 50. fig. 25), and the cilium 

 of Bacterium (Plate 1. figs. 19, 20), &c; but all the lenses made by the best makers, both 

 English and foreign, are provided with an immersion-front. These structures, which are 

 very difficult to exhibit with the older microscopes and object-glasses, show the impor- 

 tance of using very high powers, especially the T a F or ^\ inch. 



Diaphragm. — Most microscopes are provided with a diaphragm. It consists of a circular 

 blackened revolving plate, placed beneath the stage and having a series of circular apertures 

 of different sizes, each of which can be brought successively opposite to the axis of the 

 body of the microscope. It serves to regulate the quantity of light in examining trans- 

 parent objects; it also reduces the angle of the cone of the reflected rays. It is seldom, 

 however, used, nearly the same effect being produced by the two different surfaces of the 

 mirror. 



Revolving Stage-plate. — One of the plates of which the moveable stage is composed is so 

 constructed as to revolve in the same plane upon its axis, whereby an object may also be made 

 to revolve in the same manner. This apparatus, however, has some disadvantages in the 

 older microscopes ; for it renders the stage heavy and increases its depth ; and the desired 

 effect may easily be produced by rotating the slide with the fingers ; moreover it is exceed- 

 ingly difficult to place the object in the centre of rotation. It is however, provided in the 

 best modern instruments, and is by many considered of importance ; the old thickness of 

 stage has also been avoided. 



Spring Clamping-piece is intended to fix the slide upon the stage. It is of little use pro- 

 vided the slides are of the proper length, which we have given; if they are longer, the clamp 

 will prevent the accidental displacement of an object in changing the power, &c. It serves, 

 however, to fix the slide in viewing objects by oblique light, when the slide projects beyond 

 the edge of the stage, and to prevent its tilting over. 



Forceps are essential for holding opaque objects, such as insects, and viewing them in 

 different positions ; to allow of which, the handle of the forceps is made capable of 

 revolving. 



The Disk-revolver (Beck) is a very useful apparatus. It serves to bring into view all parts 

 of an opaque object, but that which is attached to the disk. 



Dark Wells are metallic cups of various .sizes, blackened inside, and serving to prevent the 

 reflection of light upon opake objects from below. They are supported in a holder, 



