POSITION OF OBSERVKll. 257 



to the beginner no better advice can be given. It is 

 simply impossible for the novice to study the correc- 

 tions of a high angled object-glass by centrally disposed 

 illumination. 



In the selection of the ohjects recommended to the 

 student, we have been governed by these and similar 

 considerations. All of the slides named contain frus- 

 tules requiring the use of light at least 40 from axis, 

 and the student will in his first attempts be compelled 

 to go considerably higher than that tigure. The exact 

 amount of obliquity essential to the best display he can 

 possibly make, will depend on his object-glass the cor- 

 rectne&s of its adjustment, and the education of his 

 eye. 



We shall, on some succeeding page have a few words 

 to say as to the management, etc., of high balsam 

 apertures by centrally disposed light. With this one 

 exception only, anything that we may have to offer in 

 the way of instruction bearing on the use of these 

 glasses will be confined to the study of the diatoms 

 which have been presented. The student who can 

 properly display the seven mounts to which his atten- 

 tion has been invited, together with the Moller plate 

 and the poclura, can safely be allowed to shift for him- 

 self. We, however, fear that our ability to teach this 

 much will not now keep pace with our sincere desire. 

 Competent, or incompetent, we propose to try. A 

 singular fact it is, that among all the works thus far 

 written on the microscope not one word of instruction 

 can be found as to the management of objectives of 



Microscopy 17 



