148 



Animal Micrology 



the camera. If the field is reduced or unevenly lighted, the camera is 

 too near or too far from the ocular, or it is tilted, or the prism is not 

 properly centered. 



Fig. 62 represents a simpler form of camera lucida with Abbe prism; 

 the mirror is fixed and close to the prism. 



Compensating Ocular. A specially designed eyepiece for use with 

 apochromatic lenses. It was found advantageous to undercorrect the 



objective and then to rectify the aber- 

 ration by over-correcting the ocular. 

 The so-called searching ocular is a low- 

 power compensating ocular used for the 

 first finding of objects. The object once 

 located in the field, the higher working 

 oculars are used in observation. 



Condenser. A lens or a series of 

 lenses mounted in a substage attach- 

 ment for the purpose of concentrating 

 light upon the object to be examined. 

 They are made in various grades of ex- 

 cellence non-achromatic, achromatic, and 

 apochromatic. Some wide-angle con- 

 densers are used as immersion conden- 

 sers; the immersion fluid is placed between the upper surface of the 

 condenser and the lower surface of the object slide. Condensers are 

 especially valuable with high-power objectives and oil-immersion lenses. 

 They are constructed to receive parallel rays of light, hence the plain' 

 mirror only should be used with them if the illumination is from day- 

 light. See illumination. 



Correction Collar. A device for adjusting the distance between the 

 lens systems of objectives so that the proper corrections may be made 

 for different thicknesses of cover-glass. Low-power objectives are not so 

 sensitive as those of high power to the influence of the cover-glass. 

 Ordinary objectives, however, are mounted in a rigid setting and cor- 

 rected for a specific tube-length and a standard cover-glass (about 

 0.18 mm. thick). With a cover-glass of different thickness correction 

 should be made by altering the tube-length of the microscope, lengthen- 

 ing it for a thinner cover and shortening it for a thicker one. With homo- 

 geneous immersion lenses the defect caused by different thicknesses of 

 cover-glass disappears (see immersion objective). See also tube-length. 



Cover-Glass Correction. See correction collar. 



Definition. The power of a lens io give a clear, distinct image and 

 make visible minute details. See resolriinj /><>trcr. 



FIG. 62. Abbe Prism. 



