PRACTICAL EXERCISES WITH THE MICROSCURE 251 



9. Condenser and Different Slide Thicknesses. — Have 

 a dry or water-immersion condenser, corrected or adjusted 

 for standard lamp distance, and for a slide 1 millimeter 

 thick. Observe the maximum aperture of the condenser 

 hght circle at the back of the objective (of 1.3 or 1.4 aper- 

 ture), with a 3-millimeter diaphragm on the source of light, 

 and a standard slide, 1 millimeter thick. Also the absence 

 of a marginal ring on either raising or lowering the con- 

 denser. Shift to a sHde about 0.8 miUimeter thick, and 

 note that the aperture of the refocused condenser is cut 

 down, and that a marginal ring appears on raising the 

 condenser, showing undercorrection. Try the same with 

 a slide about 1.2 millimeters thick. Here the aperture is 

 also lessened, but the marginal ring appears on lowering 

 the condenser, showing overcorrection. (Coles.) 



10. Condenser and Different Lamp Distances. — Use 

 the same condenser and objective as in 9, adjusted with the 

 illuminated ground glass, used as the source of light, at 25 

 centimeters distance. Shift the source to 15 centimeters, 

 and observe the diminution of the maximum condenser 

 aperture after refocusing, and the appearance of a marginal 

 ring on raising the condenser, showing undercorrection. 

 If the source is changed to 40 centimeters, the effects of 

 overcorrection are visible. (Nelson, Coles, Hartridge.) 



11. Uncorrected Condenser with Condensing Lens. — 

 With a large circle of illuminated doubly ground glass 

 before a lamp of 100 or 150 watts, a large uncorrected 

 condensing lens is arranged, by shifting its distance and the 

 distance of the lamp, to throw an image of the illuminated 

 ground glass just below the lower lens of the uncorrected 

 condenser, large enough to fill the lens with light. The 

 plane mirror is used. The condenser should be racked 

 up near the hmit. The aperture should be altered by the 

 iris. Intensity should be decreased by appropriate screens. 

 The aperture is to be tested by looking at the condenser 

 Hght circle on the back of a high objective. Glare cannot 

 well be avoided with high apertures, because of the lack of 

 correction. (Koehler.) 



