398 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. 402). 



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- 



