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 cause disagreeable reflections and shadows. 



In medium and high powers, the front lenses will 

 be so clcTse to the water, urine, blood, etc., that capillary 

 attraction will cause an adhesion to the front surface 

 of the objective ; besides this, there is such a consider- 

 able 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 sharp focus 

 becomes impossible. By merely dropping a cover 

 glass upon it all these objections are overcome. 



Covers are commercially classified as No. i, No. 2 

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

 different numbers. The variation is about as follows : 



No. i, 1-150 to 1-200 inch, or o.i 6 to 0.13 mm. thick. 

 No. 2. i-ioo to 1-150 inch, or 0.25 to o.i 6 mm. thick. 

 No. 3, 1-50 to i-ioo inch, or 0.50 to 0.26 mm. thick. 



According to the 'prices of cover glasses, when 

 purchased by weight, the No. i gives the greatest 

 number and No. 3 the least. It may for this reason 

 be thought that the purchase of No. i is most advant- 

 ageous, but it must be considered that there is a 

 greater amount of breakage by cleaning, as they are 

 very thin and sensitive. Considered from the optical 

 standpoint the No. 2 covers offer a range in thickness 



141 



