142 



Chromatic Aberration. This may be judged 

 as described under " Chromatic Aberration " in a 

 a previous chapter. 



- 



Cover Glass. We have thus far not considered 

 the cover glass, except to show its influence on the 

 optical performance of objectives. In preliminary 

 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 reflections and shadows. 



In medium and high powers, the front lenses 

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

 capillary attraction will often cause an adherence 

 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 impos- 

 sible. By merely dropping a cover glass upon it 

 all these objections are overcome. 



The covers are commercially classified as No. 1, 

 No. 2 and No. 3, but there is a variation within the 



