MICROSCOPIC DRAWING AND ENGRAVING. 



The motion of an homogeneous liquid in tubes completely filled 

 with it could not be made sensible to the sight ; but on the other 

 hand, that of a liquid containing solid particles suspended in it, 

 continually entering into collision with and displacing each other, 

 would be perfectly visible. 



The blood therefore contains certain solid particles floating in 

 and circulating with it, to which moreover are due several of its 

 most important properties ; these particles exist in countless num- 

 bers, and of minuteness so extreme, that a single drop of blood, 

 no larger than might be suspended from the point of a needle, 

 contains myriads of them Until recently, observers recognised 

 only one species of the corpuscles, such being the only ones per- 

 ceivable by the ordinary methods of observation, and being 

 incomparably more numerous than the others, which, besides 

 being more rare, are generally hidden by the former / which com- 

 pletely fill the field of the microscope. 



72. These sanguineous corpuscles are distinguished by regular 

 and constant forms, by a complex composition and a determinate 

 structure. They possess a real organisation, and pass through a 

 regular succession of phases, having a beginning, a development, 

 and an end. 



They consist of three species : first, red corpuscles ; secondly, 

 white globules ; and thirdly, white granular particles, much 

 smaller, to which observers have applied the name " globulines." 



73. Nothing can be more simple or more facile than the method 

 of observing the first class of these corpuscles. Take a sharp 

 needle and prick with it slightly the end of the finger, so as to 

 draw the smallest drop of blood ; having previously rendered a 

 small slip of glass perfectly clean and dry, touch it with the 

 blood, a small portion of which will adhere to it, and upon this 

 lay a thin film of glass, such as are prepared by the opticians for 

 microscopic use, so as to flatten between the two glasses the small 

 drop of blood. Let the glass thus carrying the blood be placed 

 under a microscope having a magnifying power of about 400 ; a 

 multitude of the red corpuscles will then be immediately visible, 

 distributed irregularly over the field of view of the instrument. 



Fig. 38, p. 81, has been reproduced from one of Dr. Donne's 

 engravings ; it represents a thin disc of human blood, having a 

 diameter equal to the 120th part of an English inch, included 

 between the two glasses. 



The red corpuscles alone are here visible ; their form is that of 

 flat discs a little concave in the middle, swelling upwards towards 

 the edges, which are slightly rounded. Some of them, such as 

 a a a, are presented with their flat sides to the line of sight, so 

 as to show very distinctly their form ; others, such as b b t are 

 100 



