146 Animal Micrology 



Apochromatic Objective. An improved form of objective which is more 

 exactly achromatic than the ordinary objective because it is corrected for 

 rays of three colors instead of two, and this correction is equally good in 

 all parts of the field. In the ordinary achromatic objective after correc- 

 tion there is a residue of color which is known as the secondary spectrum. 

 In the apochromatic lenses correction is made for a third color, and 

 usually only a slight tertiary spectrum is left uncorrected. Spherical 

 aberration is also more fully corrected. Furthermore, in these objectives 

 the foci of the optical and the chemical rays are identical, hence the 

 lenses are well adapted to photography In the glasses of the apochro- 

 matics, silicon is replaced by boron in the flint series, and by phosphorus 

 in the crown series. Fluorite was used in conjunction with the glasses 

 in the earlier forms of apochromatic lenses, with the result that the 

 lenses frequently deteriorated in warm, moist climates. Several makers 

 are now able to construct apochromatic objectives without the use of 

 fluorite. Both dry and immersion apochromatics are made. 



Binocular Microscope. A microscope adapted to vision with both eyes 

 at once. By means of a prism part of the light from the object is 

 diverted into a second tube which like the main tube is provided with an 

 eyepiece. Binocular eyepieces for attachment to an ordinary microscope 

 are now made. Binocular microscopes yield a stereoscopic view so that 

 objects which have any amount of depth stand out in relief exhibiting 

 their natural contour. The instruments can be used successfully only 

 with objectives of comparatively low power. 



Brownian Movement or Pedesis. An oscillating or dancing motion ob- 

 servable in small particles in a liquid when seen under the microscope. 



Camera Lucida. An apparatus containing a glass prism or thin glass 

 plate so arranged that when placed over the eyepiece of the microscope 

 the observer may see the image of the object under the microscope pro- 

 jected on to his drawing-paper on the table. The point of the pencil is 

 also visible, consequently the outline of the object may be readily traced 

 on the paper. In the simpler camera lucidas a thin neutral tint glass 

 slip is so arranged that it is in alignment with the eye-lens of the ocular, 

 except that it sets at an angle of 45 to it. When the microscope is 

 tilted into a horizontal position the observer sees the image of the object 

 reflected from the upper side of the glass slip, but. since the latter is 

 somewhat transparent, he also sees the white paper spread below on the 

 table (Fig. 60). 



Another form of simple camera lucida is the Wollaston. To use 

 it the microscope must be inclined. The essential part of the camera 

 consists of a quadrangular prism. The eye of the observer is so placed 

 over the edge of the prism as to receive rays of light from the object 



