APPENDIX. USE OF THE MICROSCOPE. 205 



upper surface, but not too thick to admit of considerable obliquity of illumina- 

 tion, and it should not be less than about three inches wide. 



5. The coarse adjustment had better be by sliding tube, as the rack-and- 

 pinion when well made is expensive, and when not well made is very liable to 

 get out of order and render the stand next to worthless. For the fine adjust- 

 ment a micrometer screw is necessary, and this should be very carefully con- 

 structed so that it will last well and produce no lateral motion even when used 

 with the highest powers. 



6. The stand should afford room enough between the Ftage and the front 

 end of the tube, when the latter is drawn out to its extreme limit, to admit of 

 the use of an objective of as low a power as a three-inch, if desired. 



7. While most of the work of the student will be done with the tube 

 shortened to six and one-eighth inches (155 mm.), as in the continental model 

 stands, it is a convenience if the body be provided with a draw-tube, so that it 

 may be extended to ten inches, the standard English length of tube. The 

 magnifying power may then be varied within certain limits without changing 

 the optical parts. 



8. There should be a diaphragm beneath the stage, for the purpose of 

 increasing or diminishing the amount of light admitted to the objective. 



9. The reflecting mirror beneath the stage should have a concave surface 

 whose focus is not far from the level of the stage, and the mirror itself should 

 have a diameter of not less than an inch and a fourth. It should also be so 

 mounted as to be capable of convenient adjustment at any angle. It is an 

 advantage, also, if the mirror is mounted on an* arm or bar capable of swinging 

 vertically on an axis which lies in the same plane as the stage (see illustration), 

 so that light may be directed upon the object from any point below or above 

 the stage. Such an arrangement saves the additional expense of a bull's-eye 

 condenser for the illumination of opaque objects. 



10. The optical parts, both objectives and eye-pieces, should be of good 

 construction. The form of eye-piece called the Heyghenian is the best for 

 regular work. The objectives should give a flat field, clear and sharp definition 

 of the object, and be practically free from color aberration, even when used 

 with an eye-piece of one inch focal length. 



11. Much the larger proportion of all the work the vegetable histologist 

 is called upon to do may be done with powers ranging from thirty to about 

 seven hundred diameters. No higher powers than these are necessary for the 

 work laid out for the student in these chapters. These may be obtained, 

 approximately, by means of two eye-pieces, one having an inch and a half and 

 the other an inch focal length, and two objectives, the lower power having a 

 focal length of one inch or two-thirds inch, and the higher a focal length of 

 one-sixth or one-eighth inch. 



ACCESSORY APPARATUS. 



The following may be regarded as necessities : 



A pair of delicate forceps or tweezers for handling sections, minute pieces of 

 tissue, etc. 



A pair of sharp-pointed scissors. 



