WORK OVER DRY MOUNTS, ETC, 



303 



podura, and as we have stated, this slide must necessa- 

 rily be selected by some friend competent to the task. 

 The American podura is quite inferior to the English, 

 yet of the two a slide of the very best American 

 should be accepted rather than a notably poor English 

 specimen. The former will be much better than none 

 at all, but let it be remembered that the English are 

 the desired thing. Now as to the proper appearance of 

 these scales under the objective, we must state that it 

 is perfectly impossible to give any truthful representa- 

 tion on paper. The following cut 

 taken from Prof. Phin's book, 

 copied irom the illustrated cata- 

 logue of the late Richard Beck, 

 may help matters some, and shows 

 the podura with tolerable precision, 

 when the same is seen with a dry 

 one-eighth, say of 130 aperture 

 magnified 1,300 diameters, and illuminated with light 

 as central as possible. Now let the student take a half 

 sheet of writing paper and roll this up so as to form a 

 tube, and examine the cut attentively, looking through 

 this tube, as is frequently done when examining draw- 

 ings, etc. Letting a strong light fall from the direc- 

 tion indicated by the shadowed sides of the markings, 

 we think that most of our readers will have no diffi- 

 culty in noticing that the " exclamation points " seem 

 to " rise up " above (apparently) the general surface of 

 the scale. Our principal design in calling attention to 

 this feature is that he may thus recognize an effect to 



