112 



PLANT ANATOMY. 



Sphorical aberration is a second defect 

 whereby all points of an image are not 

 brought together, so that in looking at a 

 piece of fine wire netting, for instance, 

 the fibers in the centre are straight and 

 distinct, while those on the outside are 

 found to be curved and blurred. 



r 



A certain amount of spherical aberra- 

 tion is necessary in high power objec- 

 tives, but by means of the diaphragm 

 the outer portions can be cut out of 

 view. 



Most modern objectives are carefully 

 corrected for both these errors, but they 

 should be remembered by the purchaser. 



Good lenses should also possess good 

 defining power, that is, the image should 

 be distinct, especially at its outer bor- 

 ders; the diaphragm has much to do 

 with the distinctness of the image, and 

 care should be taken to limit the amount 

 of light when one is testing a lens for 

 its defining power. 



Flatness of field must vary with the 

 magnifying power and angle of aperture 

 of the lens, A flat field is one in which 

 all the parts of the field are in focus at 

 the same time. Regarding the angle of 

 aperture little need here be said, save 

 that it represents the pencil of light that 

 the lens is able to take in and use in 

 forming an image; and that the angle of 

 aperture of a lens more clearly tells its 

 powers than the tables of magnification. 

 Wide-angled lenses that are properly 

 corrected are to be preferred. 



Good working distance is of great im- 

 portance to the microscoplst in the phar- 

 macognostical laboratory, and such ob- 

 jectives should be chosen that have this 

 quality, even if a certain amount of 

 magnification should be sacrificed. It 

 means that tliere should be an apprecia- 

 ble distance between the front lens and 

 the object. In low-power objectives it 

 is of little concern, but In the higher 

 powers it becomes important, especially 

 to the pharmacist who may desire to 

 look at some crystals in a fluid in a 

 watch crystal; if he has a lens with a 

 short working distance it may be impos- 

 sible, because the objective will not 

 focus unless stuck into the liquid, and 

 this destroys the image, whereas a long 

 working distance lens would give the 

 requisite ability to manipulate the ob- 

 ject. 



pharmacognocist knows how to use the 

 microscope, but the following brief di- 

 rections may be of service to the begin- 

 ner. 



The working table should be firm and 

 is best about 28 or 30 inches in height. 



The light from an illumined cloud is 

 the best light to be used and most mi- 

 croscopists prefer north light. Any light 

 can be used, but not too much. When 

 using a lamp the light from the ground 

 glass globe is preferable to that from 

 the wick. 



The microscope should be placed di- 

 rectly in front of the observer, with the 

 pillar facing backward. To get the light 

 the mirror, preferably the concave side, 

 should be so turned as to throw the 

 beam directly up through the aperture. 

 All v'ork shoiiJd he hcfjnn with theJoW' 

 Voiicr objective. Having obtained the 

 illumination of the object, which may 

 bo mounted on a slide or held in a glass 

 evaporating dish, this object is placed 

 as near the centre of the stage as the eye 

 can approximate, when by means of 

 the coarse adjustment the low power is 

 lowered as near the object as possible 

 without touching it; then with the eye 

 over the ocular, the tube should be 

 gradually raised by means of the coarse 

 adjustment, until the object is in an ap- 

 proximate focus; the exact focus can 

 then be obtained by means of the fine 

 adjustment. In general it is not neces- 

 sary to turn the fine adjustment more 

 than one revolution. 



Having observed what was necessary 

 with the low power, the portion of the 

 object further to be investigated should 

 be moved so as to come in the centre of 

 the field, the observer still looking into 

 the microscope. 



The tube is then raised and the high 

 power substituted, and the process of 

 focusing, as already outlined, repeated. 

 It is essential in using the high powers 

 to exercise great care. The objective ap- 

 proaches the object as near as it can be 

 placed, and then with the eye over the 

 ocular the tube is raised very slowly by 

 means of the coarse adjustment until 

 the focus is reached; then the focus is 

 completed as before by means of the 

 fine adjustment. 



To illustrate the value of the dia- 



It i» to be taken for granted that the iV'/'"^ f Jf ^rl!!!!!!".':^!" * "^"^ ^f 



tried. A thin transparent object should 



