1885.] 



MICROSCOPICAL JOURNAL. 



145 



tube length is, perhaps, in itself open • 

 to objection, for it seems to indicate 

 tliat the lo inches must be measured 

 from some point in the objective to 

 some other point in or about the eye- 

 piece or tube. A correct understand- 

 ing of the matter woidd seem to indi- 

 cate that the tube is of such a length ■ 

 that when the eye-piece is in position 

 it brings to a proper focus for the eye 

 the rays coming from the objective, 

 which alone would form an image at 

 lo inches distance from its optical 

 centre. In other words, standard 

 conditions should mean that the ob- 

 jective is at such a distance from the 

 object that, alone, it would form an 

 image at the lo-inch distance from the 

 optical centre ; and the draw-tube so 

 arranged that the eye-piece brings 

 such rays to a proper focus for the eye. 



To practically arrange the length 

 to conform to these principles, take 

 the tube in the position in which it 

 was left after finishing the final meas- 

 urements for the optical centre, and 

 focus again carefully so that the image 

 is clear on the screen. Then remove 

 it and put in the eye-piece. If the 

 object is clearly seen, no further ad- 

 justment is necessary. If not, hold 

 firmly the coarse adjustment and ad- 

 just the draw-tube until the object is 

 properly focussed. A rri'irk on the 

 tube or a note of the length drawn 

 out fixes the position for this eye- 

 piece. For another it will probably 

 be somewhat different. Now, under 

 such conditions, when the tube, car- 

 rying objective, eye-piece, and all, is 

 focussed up and down in the usual 

 way, the objects brought into view 

 are at the same constant distance from 

 the objective, so that their images by 

 the objective alone would be formed 

 at the constant standard distance of lo 

 inches from its optical centre. 



These remarks of course apply 

 principally to the higher powers not 

 furnished with correction collar, 

 where a constant set of conditions is 

 very desirable. Light, thickness of 

 cover, mounting medium, etc., of 

 course constitute part of the condi- 



tions under which an objective is 



used, but, supposing these constinit. 

 we are here only concerned with the 

 relative positions of object, objective, 

 and eye-piece. These may be more 

 or less widely departed from by the 

 expert microscopist in order to bal- 

 ance some other unusual conditions, 

 such as an extra thick or extra thin 

 cover glass, some peculiarity in eye- 

 piece or mounting medium, &c. But, 

 aside from these, it would seem de- 

 sirable to have some general method 

 of using an objective under constant 

 conditions, so that the interpretation 

 of its appearances being once under- 

 stood, the same principles may be 

 always applied in future cases. 

 o 



Diatoms of the Gulf. 



BY J. D. COX, LL.D., F. R. M. S. 



Scientific students, and especially 

 those who are interested in the inves- 

 tigation of the diatoms, will, we are 

 sure, take pleasure in the narrative or 

 the work done by a trio of enthusias- 

 tic students in Mobile. These gentle- 

 men are Messrs. K. M. Cunningham, 

 W. S. McNeill, and G. H. Taylor. 

 Neither of them is a professional 

 botanist, all have other employments 

 to which their ordinary working 

 hours must be given, yet they have 

 laid the students of the diatoms under 

 such obligation to them that a recog- 

 nition of the value of labors such as 

 theirs is no more than their just due. 

 The last named of these gentlemen is 

 a surgeon dentist ; Captain McNeill, 

 formerly of the Confederate army, is 

 the recent commissioner of Alabama 

 to the New Orleans Exposition, and 

 Mr. Cunningham is engaged in rail- 

 way business. 



In the summer of 1 878 Mr. Cunning- 

 ham accidentally became acquainted 

 with Prof. Bailey's papers upon the 

 Atlantic and Gulf diatoms in the 

 Smithsonian * Contributions,' and a 

 zeal for research was thus awakened. 

 He began a somewhat systematic ex- 

 amination of the region about Mobile 

 for crude diatomaceous material, both 



