S INTRODtJCTIOi^. 



3. Three objectives and two eye-pieces, from combinations of 

 whicli magnifying powers of fort^' to eiglit hundred diameters 

 can be obtained, will suflice for nearlj' all the histological work 

 described in this volume. Two objectives and a single eye- 

 piece furnishing powers of sixtj" to five hundred diameters are 

 enough for all ordinary investigations of minute structure. Ade- 

 quate and convenient illumination is secured by a plane and a 

 concave mirror under the stage. If this is supplemented by an 

 achromatic condenser, so much the better. The stage, prefer- 

 ably thin, should be provided with a perforated revolving disc, 

 or other suitable sj-stem of diaphragms, by which its centi-al 

 aperture can be made larger or smaller. 



4. The student ought, at the outset of his work, to make 

 himself familiar with the principal effects which are produced 

 in the appearance of the object in the field of the microscope, 

 by changes in the amount and direction of the light thrown by 

 the mirror. Details can sometimes be brought out clearly by 

 oblique illumination, which are onl}' faintly, if at all, seen in 

 direct light. 



5. In general, low magnifying powers are to be preferred to 

 higher ones ; and combinations of high objectives with low eye- 

 pieces, securing a given magnifying power, are always better 

 than those in which low objectives and high eye-pieces are used 

 to obtain the same enlargement. 



6. The slips of glass, or "slides," upon which microscopic 

 objects are commonl3- prepared and preserved, are three inches 

 (76 mm.) long b}' one inch (25 mm.) wide. This is for most 

 cases a more convenient size than that frequently employed in 

 Germany ; namely, 48 X 28 millimeters. The glass should be 

 free from color and from imperfections. The preparation to be 

 examined under the microscope should be covered with a disc 

 of thin glass before it is brought under the objective. Perfect 

 cleanliness of sUde and cover-glass is absolutel}' necessarj- in all 

 examinations, and must be secured by the exercise of scrupulous 

 care.^ 



7. Dissecting instrumentr.. Sharp delicate needles, by whicli 



1 For cleaning glass perfectly, the following preparation may be used : — 

 A strong solution of potassic bichromate to which about half as much con- 

 centrated sulphuric acid is cautiously added. To this mixture add an equal 

 volume of water. The glass slips, or covers, are to be kept in this solution for 

 a short time, and then thoroughly rinsed in pure water, after which they may 

 be dried with cloth or wash-leather. For ordinary use alcohol of usual strength 

 answers the purpose very well. 



