WORK OVER DRY MOUNTS, ETC. 307 



As to the general examination of the various dry 

 mounts, or of histological preparations mounted in 

 glycerine, nothing but the most general directions can 

 be given. The previous study of the diatoms will 

 become a valuable assistance to the student, and this 

 study should be still kept up in the examination of dry 

 mounts especially. Of such objects as have palpable 

 size, and can be seen with the naked eye, as is the case 

 with very many histological mountings, the pupil must 

 expect more or less annoyance from the interference of 

 the various shadows, from different parts of the struc- 

 ture, situated in different planes. If, in such a mount 

 it be desired to examine critically minute details, make 

 it a point to select a bit of the object that has per- 

 chance been accidentally removed from the main body, 

 and when you prepare your own mounts keep this 

 point in mind. A good general rule with such mounts 

 is to keep the illumination as nearly central as possible. 

 In the adjustment of the objective the same plan is to 

 be adopted, selecting when available the smallest 

 isolated fragment, with the illumination within 35 

 to 40 from axis (or less) the low angled sub-stage 

 condenser will be valuable. It however requires some 

 study, as we have found by practical experience, to 

 work the condenser so as to secure its best effects. It 

 is a matter, also, that calls for the outlay of time and 

 no little patience. Nevertheless, the reward will repay 

 the effort. 



Through the kindness of Mr. Zentmayer, who con- 

 structed the apparatus we have been experimenting 



