CH. II '] 



LIGHTING AND FOCUSING 



Si 



diaphragm (Fig. 36D) so that it will project about half way across the 

 opening. If one uses this ocular, the pointer will appear in the field 

 and one can place the specimen so that the pointer indicates it exactly, 

 as in using a pointer on a diagram or on the black-board. It is not known to 

 the author who devised this method. It is certainly of the greatest advantage 

 in demonstrating objects like aruoebas or white blood corpuscles to persons 

 not familiar with them, as the field is liable to have in it many other objects 

 which are more easily seen. 



SS 



70 71 72 



Figs. 70-72. Sectional Views of the two Forms of the Marker. 



Fig. 70. The simplest form of marker. It consists of the part SS witk 

 the milled edge (M) . This part bears the society or objective screw for attach_ 

 ing the marker to the microscope. R. Rotating part of the marker. This 

 bears the eccentric brush (B) at its lower end. The brush is on the wire ( W)_, 

 This wire is eccenttic, and may be made more or less so by bending the wire. 

 The central dotted line coincides with the axis of the microscope. The revolv- 

 ing part is connected with the " Society Screw " by the small screw (S). 



Fig. 71. 55, R, and B. All parts same as with Fig. 70, except that the 

 brush is carried by a sliding cylinder the end view being indicated in Fig. 72. 



J! 141. Mechanical Stage. — For High School and ordinary laboratory 

 work a mechanical stage is not needed; but for much work, especially where 

 high objectives are used a mechanical stage is of great advantage. It is also 

 advantageous if the mechanical stage can be easily removed. 

 The one found on the most expensive American and English microscopes for 

 the last twenty years and the one now present on the larger continental micro- 

 scopes, is excellent for high powers and preparations of moderate dimensions, 

 but for the study of serial sectons and large sections or preparations in general^ 

 mechanical stages like those shown in Figs. 79-89 are more useful. This form 

 of mechanical stage has the advantage of giving great lateral and forward and 



