APPENDIX 135 



although difficult to obtain, while an achromatic triplet of sufficient power will probably 

 be all that is necessary. For selecting with the mechanical finger, an objective of two- 

 thirds-inch focus is the most convenient, but for determining species a one-fifth-inch is 

 needed, an immersion objective being essential for minute forms. 



No particular form of microscope is required. Any instrument having standard parts, 

 inclination of the body to the axis, a sub-stage condenser and movable stage, will prove 

 serviceable in nearly all investigations. For critical work, measurement of striae and 

 location of specimens on the slide, the large models of Bausch and Lomb leave nothing to 

 be desired. One smaller instrument may be used for rapid examination and for selection 

 with the mechanical finger. If the stage is supplied with a vernier, the diatoms can be 

 located rapidly and recorded for future reference. The Zentmayer Army Hospital stand 

 with mechanical stage is excellent. The Continental stands, convenient for laboratory 

 work, especially in the examination of bacteria, are not so serviceable as the larger 

 stands of American and English make. The stand especially designed by Dr. Henri 

 Van Heurck, the celebrated Belgian naturalist, is, without doubt, admirably suited to 

 the investigation of the Diatomaceae. In the form of the Circuit Stage as made by 

 Watson and Sons, of London, supplied with proper condenser and mechanical stage with 

 vernier attachment, it has been used in the preparation of the present work with much 

 satisfaction. 



The drawings have all been made with an Abb6 camera lucida, a 3 mm. objective and 

 a No. 10 eye-piece, producing a magnification of about 800 diameters. All illustrations are 

 from actual specimens in my cabinet or, in a few instances, from slides sent me by friends. 

 In the measurement of striae and puncta, the number in ten microns is stated, and will be 

 found to be approximately correct in most of the drawings, except when the number is in 

 excess of twenty in ten microns, in which case it is impossible to represent the markings 

 accurately on figures of the magnification adopted. All drawings are from specimens in 

 this locality, except in a few cases mentioned in the text. 



