62 



to the angle of incidence. An opaque stop, which 

 is cemented to the concave surface, prevents the 

 light from passing through the central portion of the 

 paraboloid. The object is thus illuminated on all 

 sides by such an obliquity qf light, that it does not 

 pass into the objective ; the object stands out in 

 relief, pleasantly illuminated on a dark back-ground. 

 In using the paraboloid, the plane mirror should be 

 used, and it is .necessary to vary its distance from the 

 object in order to attain the best results. 



Cover-Glass. Thus far no attention has been 

 given to the use of the cover-glass, although it is an 

 important factor in reaching good -results. In prelim- 

 inary examinations of solid objects with low powers 

 it may be dispensed with ; but where fluids are used, 

 whether with low, medium, or high powers, it should 

 always be used. A drop or small quantity of fluid 

 placed upon a slide assumes a spherical form, and, on 

 viewing it with a low power, it will be found to give 

 a distorted field, and will cause disagreeable reflec- 

 tions and shadows. 



As stated before, medium and high powers have a 

 comparatively short working distance, and the front 

 lenses will be so close to the water, urine, blood, etc., 

 that the capillary attraction will often cause an adher- 

 ence to the front surface of'the objective; besides 

 this, there is such a considerable depth to the fluid 

 that it obstructs the light, requires a great change in 

 adjustment for the various planes, and is usually in 

 such vibration that a sharp focus becomes impossible; 



