296 HOW TO SEE WITH THE MICiiOSCOPE. 



plate passing from No. 19 to 20 is quite a jump. One 

 may see the No. 19 in a very satisfactory manner, and 

 yet fail of the No. 20. At the onset it will be better 

 to secure all the obliquity of light possible. Should 

 the Acme or the little Histological stand be used, the 

 upper one-third of the mirror may rise above the stage, 

 the lamp and condenser to be adjusted with the greatest 

 nicety. If the No. 20 resist the attack, it is preferable 

 not to spend much time over it, but go at once back to 

 the No. 19 and endeavor, by some careful re-adjustment, 

 to get a still better view of this ; and when satisfied that 

 such has been accomplished then return to the attack 

 on the No. 20. One will always succeed much faster by 

 trying to improve what is seen than by blindly working 

 over an object of which the desired details cannot be 

 displayed at all. The ease with which the learner can 

 run from one test to another renders the Moller plate 

 very acceptable to the learner. Should it occur that 

 the No. 20 make a protracted resistance, let the above 

 programme be adhered to, spending nearly all of the 

 time in perfecting the display of the No. 19. It may 

 be worthwhile to add that the Moller Amphipleuras are 

 about on a par, in point of difficulty, with those from 

 Aberdeen, Scotland. If tiiere be any difference, my 

 impression is that of the two the Moller No. 20 are 

 perhaps the easier test. 



In studying the Aberdeen, or in fact any mount con- 

 taining severe tests, it is always advisable to correct the 

 glass approximately by selecting some fine object, of 

 which the details can be displayed by such approximate 



