52 



LIGHTING AAD FOCUSING. 



[CH. II. 



To find the critical angle in the denser of two contiguous media : — 



Make the angle of refraction (i. e., the angle in the rarer of the two media) 



/sin A /index r\ _ , ,, . 



oo° and solve the general equation : I — — 1 = 1 .—^— . I . Let the two substances 

 ° ^ ysin /'/ \ index if 



be water and air, then the sine of r (90 ) is 1, the index of air is 1, that of water 



1.33, whence { ) = ( — \ or sin / = 751 +. This is the sine of 48°+, and 



whenever the ray in the water is at an angle of more than 48 it will not emerge 

 into the air, but be totally reflected back into the water. . 



The case of a ray passing from crown glass into water : 



/ sin i \_ / index water (1.33U Qr / sin A _./i.33\. 



\sin r (sin 90°= 1)/ \ index glass (1.52)/ \ 1 / \i.52/ 

 whence sin i = .S75 sine of critical angle in glass covered with water. The cor- 

 responding angle is approximately 61 °. 



\ 94. Color Irnag-es. — These are images of objects which are strongly colored, 

 and lighted with so wide an aperture that the refraction images are drowned in the 

 light. Such images are obtained by removing the diaphragm or by using a larger 

 opening. This method of illumination is specially applicable to the study of 

 stained microbes. (See below, | 101). 



ADJUSTABLE, WATER AND HOMOGENEOUS OBJECTIVES. 



EXPERIMENTS. 



\ 95. Adjustment for Objectives. — As stated above {\ 22), the aberration pro- 

 duced by the cover-glass (Fig. 56), is compensated for by giving the combinations 

 in the objective a different relative position than they would have if the objective 

 were to be used on uncovered objects. Although this relative position cannot be 

 changed in unadjustable objectives, one can secure the best results of which the 

 objective is capable by selecting covers of the thickness for which the objective was 

 corrected. (See table in I 27). Adjustment may be made also by increasing the 

 tube-length for covers thinner than the standard and by shortening the tube-length 

 for covers thicker than the standard (Fig. 57). 



Fig. 56. — Effect of the cover-glass on 

 the rays from the object to the objective 

 {Ross). 



Axis. The projection of the optic 

 axis of the microscope. 



F. Focus or axial point of the objec- 

 tive. 



F / and F" . Points on the axis where 

 rays 2 and j appear to originate if 

 traced backward after emerging from 

 F the upper side of the cover-glass ( Cover) . 



In learning to adjust objectives, it is best for the student to choose some object 

 whose structure is well agreed upon, and then to practice lighting it, shading the 

 stage and adjusting the objective, until the proper appearance is obtained. The 

 adjustment is made by turning a ring or collar which acts on a screw and increases 

 or diminishes the distance between the systems of lenses, usually the front and the 

 back systems (Fig. 40). 



General Directions. — (A) The thinner the cover-glass the further 

 must the systems of lenses be separated, i. e., the adjusting collar is 



