XVI 



INTRODUCTION. 



that if Schleiden were to try to obtain a view of the hexagonal structure of the dots on the 

 valves of a Gyrosigma with his object-glasses, he would signally fail ; for the exhibition 

 of this structure requires a power of about 2000 diameters to render it distinct beyond 

 dispute, with the use of stops in the condenser unless the object-glass be of very large 

 angular aperture ; and yet it is clearly shown with the English glasses, and the fact of its 

 impressing its own image upon photographic paper, at once shows its reality, or that its 

 perception is not the result of the imagination. The origin of these results and statements 

 is explained above. 



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

 blackened plate, placed parallel to and beneath the stage, and furnished with a revolving 

 plate 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 transparent 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 has, however, greater disadvantages 

 than advantages, 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 exceeding 

 difficult to place the object in the centre of rotation. 



Spring Clamping -piece is intended to fix the slides upon the stage. It is of no 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. 



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

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



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

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

 moveable in an arm, which is inserted into some part of the stand of the microscope. Their 

 purpose is equally well effected by a slide, upon which a piece of black velvet has been 

 fastened by marine glue. 



Achromatic Condenser. This consists of an achromatic object-glass or set of lenses, in 

 an inverted position beneath the stage, moveable in all directions in its own plane and in the 

 direction of its axis. It serves to condense the light reflected by the mirror to a focus upon 

 the object, and to exclude all extraneous light. It is essential in examining minute objects 

 with high powers ; in fact, the structure of many objects cannot be made out without it. 

 In its most improved form, a rotating diaphragm is placed behind the back glass of the com- 

 bination forming the object-glass, perforated with a series of apertures of various sizes, some 

 of them being circular whilst others are annular ; the former diminishing or increasing the 

 cone or pencil of 'rays reflected from the mirror by excluding the lateral rays, the latter 

 admitting only the lateral rays, the central ones being intercepted by the portion of the 

 diaphragm within the ring, so that the angular inclination of the transmitted rays may be 

 increased or diminished at will. The markings upon many of the Diatomaceae can only be 

 made out when examined by oblique light, as procured by intercepting the central rays, 

 which effect is produced by this modified achromatic condenser. The same effect may be 

 produced in one of the achromatic condensers of the old form, provided the compound 

 lenses of which the object-glass in the condenser consists are separable (which should 

 always be the case), by pasting or temporarily placing a circular disk or " stop " of black 

 paper exactly upon the centre of the plane face of the innermost combination. The diameter 



