280 HISTOLOGICAL TECHNIQUE. 



In studying a specimen always use the lowest power that will 

 reveal the structures it is desired to see; and, in any event, use 

 a low power first, to get a general idea of the topography of the 

 specimen. In this way the portions for more minute study can be 

 readily selected, with a great saving of time. 



The proper illumination of the specimen is just as important as- 

 careful focussing. If the Abbe condenser is in use, employ the 

 plane surface of the mirror during the day ; either the plane or the 

 concave surface when artificial light is used, selecting the surface 

 which causes less glare. The iris-diaphragm should be kept ad- 

 justed so as to give the best definition of the specimen under exam- 

 ination when the latter is in focus. It will be found that when 

 colorless objects are examined a small opening gives the clearest 

 picture, while with colored objects a larger opening is preferable. 

 A small diaphragm serves to bring out the "structure-picture" ; a 

 large diaphragm, the " color-picture " (see p. 284). 



A bottle of oil of cedar-wood, having approximately the same 

 refractive index as the glass from which the cover-glasses are made, 

 is furnished with the immersion-objectives. When these are used 

 a drop of this oil is placed on the cover, and the end of the objec- 

 tive immersed in this drop. This arrangement permits the light to 

 pass from the object to the bottom lens of the objective without sen- 

 sible refraction, increasing the amount of light entering the objec- 

 tive, the sharpness of definition, and the purity of the color-picture. 

 When the lens has been used the oil should be removed with a soft 

 cloth or Japanese paper. The oil on the cover may be wiped off at 

 once, or it may be allowed to dry and then removed with a cloth 

 moistened with xylol. 



Microscopical Measurements. — These may be made, with a fair 

 degree of accuracy, by means of an eye-piece micrometer-scale. 

 This is a ruled disc of glass that can be placed upon the diaphragm 

 within the ocular, where its scale should be well defined when seen 

 through the upper lens of the eye-piece. Special micrometer ocu- 

 lars are made which permit of focussing the scale, but these are 

 unnecessary if the diaphragms of the ordinary oculars are in the 

 right places within the eye-piece tubes. The value of the divisions 

 of the eye-piece micrometer-scale must be determined by comparing 

 it with the scale of a micrometer-slide which is placed upon the 

 stage of the microscope. These scales usually consist of 1 mm. 

 divided into hundredths, and the eye-piece scale will have dif- 



