APPENDIX. 349 



The above directions are sufficient for copying a drawing or for drawing a 

 flat object but where the object has much depth, unless one be already a 

 trained artist he should draw comparison sketches. 



Comparison sketches are made of the same size but from a point of viev 

 ninety degrees apart. They are drawn in the manner just described, and com- 

 pared by meanm of a series of parallel lines. Every point in each drawing 

 should compare with the corresponding point in two adjacent figures. 



When comparison drawings fail to agree the one in wnich the object is 

 seen most nearly perpendicularly is likely to be the more accurate. In any 

 case a careful inspection should show which drawing needs to be changed. 



USE OF HAND LENS. 



Insects are so small that the use of a magnifier is essential in their study 

 and every student should be provided with a good pocket magnifier. A 

 % inch (18 m.m.) or a V2 inch coddington is recommended as a good quality 

 cheap lens and a triplet where one wants the best lens available. The 

 fractional designation of lenses indicates the magnification. The normal 

 focus of the eye is assumed to be ten inches (250 m.m.) and a lense that 

 magnifies twenty times is called a i/^ inch lens because the lens causes 

 the rays from the object to enter the eye as the the object were but half an 

 inch away, the pupil remaining adjusted for the normal ten inch vision. 



In selecting a lens avoid any showing serious aberration. Aberration is of two 

 kinds, chromatic and spherical. Chromatic aberration may be detected by first 

 putting the lens very close to a printed page and slowly withdrawing it. As 

 the letters begin to lose their distinctness, more or less color will be seen 

 around them. A lens should be rejected if these rainbow colors are very 

 pronounced. Spherical aberration may be seen when the lens is removed 

 still further from the page when the letters will appear distinct again but 

 small and upside down. If the lines are straight and not closer together at 

 the ends the lense is of good quality in this respect. 



The most important thing in the use of a lens is to see that the point at 

 which one desires to look is in the light. Do not allow the head or the lens 

 to throw a shadow on that which you are endeavoring to examine. The 

 next most important thing is to hold all steady. The best plan is to lay the 

 two hands against each other, one holding the object, the other the lens 

 and bring the cheek against the hand with the eye as close to the lense as 

 possible. 



USE OF MICROSCOPE. 

 A microscope provides for the use of higher magnification than a hand 

 lense by provisions for better illumination of the object and for greater 

 steadimess. The object is usually so prepairer that light can pass thru the 

 tissue, and it rests on a stage, the lenses being brought into focus by a screw 

 adjustment. In using a microscope observe the following points. 



