32 



THE MICROSCOPE 



Variation 

 of light 

 intensity 

 when iris 

 diaphragm 

 of condenser 

 is altered. 



Double 

 wedge light 

 moderator. 



I 



different apertures when it is required to observe very fine struc- 

 ture. 



There is a difficulty in making use of different apertures because 

 the light varies so greatly by the change in the aperture of the 

 condenser, that the variation in intensity becomes a serious incon- 

 venience. The observer should never be tempted to overdo the 

 brilliancy of the illumination ; several pieces of ground glass should 

 be at hand to place between the lamp and the mirror to modify 

 the light as the condenser is opened up. 



The most satisfactory appliance for critical work is an adjust- 

 able pair of neutral tint glass wedges mounted on a stand which 

 can be placed between the light and the mirror of the microscope 



to vary the intensity of the 

 light at will. The apparatus 

 consists of a frame which 

 carries two neutral glass 

 wedges which slide in fit- 

 tings and are connected 

 together so that they move 

 over one another in oppo- 

 site directions by sliding 

 a knob. The total thick- 

 ness of the neutral tint 

 glass is varied and the 

 brilliancy of the illumina- 

 tion increased or decreased 

 within very considerable 

 limits. The frame is at- 

 tached by means of a clamp 

 to a rod fixed to a strong 

 stand. Its height from the 

 table may be varied from 

 2 inches to 8 inches, and 

 two slides are provided in 

 front of the wedges for the 

 reception of colour filters or 

 ground glass. 

 As the illumination is increased by opening the diaphragm 

 of the condenser, it can be reduced by thickening the neutral 

 tint layer of glass by sliding the two wedges over each other. 

 This apparatus can be provided with wedges of different intensities 

 according to the strength of the illuminant with which it is to 

 be used. 



The use of a small bundle of oblique light directed upon the 

 object at a particular angle has been studied in connection with 

 the delineation of line structure. It is accomplished by cutting 

 a small hole in an opaque sheet of card, metal, or celluloid, and 

 placing this aperture in different positions under the condenser, 



Fig. 21.— No. 3328, Double Wedge 

 Moderator. 



