On Drmving Prisms. By Dr. Anthony. 699 



angtilatum, as executed with various prisms, the sketch of each 

 having occupied as nearly as possible half a minute ! Those who 

 have sketched P. angulatum will be conscious that several minutes 

 are generally needed to get the peculiar curves of this diatom 

 satisfactorily. Here are many outlines made in succession of this 

 same lorica, to show how identical they all are in character. The 

 large drawing or diagram of some well-known forms of diatoms 

 is a tour de force, and here the effect would be better, or easier 

 got, by copying s(jme moderate-sized prism outlines, by means of 

 the pantograph ; but the drawing as it stands really was executed 

 under the Microscope ; the paper was laid on the ground with a 

 bright light thrown upon it, the Microscope was well raised over the 

 edge of the table. The image — enormously amplified — got from 

 a 1/6 objective, was projected by the prism, and was traced upon 

 the paper by the aid of a pencil ; which pencil might be said to be 

 some 5 feet long, inasmuch as it was formed by a crayon tied 

 firmly on a joint of a fishing-rod. That the outline so traced was 

 a bit " shaky," and needed " mending " may well be imagined, but 

 the reparation has, 1 think, not been made at the expense of the 

 characteristic curves of the various diatoms. I am sure the relative 

 size of each may be depended on, though I must own to depicting 

 the largest and boldest specimen of P. angulatum I could lay my 

 hands on. 



After this much of preamble, permit me to name the various 

 forms of drawing appliances or prisms from which to make a 

 selection ; all of them have some merit, and some of them, as I 

 trust I shall be able to show, are pre-eminently useful. First 

 come " steel disk " or " neutral-tint " glass reflectors ; then prisms 

 proper, by "WoUaston, Gundlach, Beck, Oberhauser, Zeiss, Nobert, 

 Abbe, Nachet, and Schroder. They may be classed thus, 

 Wollaston, steel disk, and glass reflector can only be used when the 

 Microscope is placed horizontally — a position which is always a 

 more or less cramped one for the observer, and which is all but 

 impossible to adopt in connection with dissections, and indeed with 

 most objects mounted in fluid and more or less free to move in the 

 cells. The prisms of Beck, Gundlach, Zeiss, and Schroder are avail- 

 able when the Microscope is set at the usual observing angle. 

 The prisms of Oberhauser, Nobert, Abbe, and Nachet can only 

 be employed when the Microscope is placed in a vertical position ; 

 the image is projected a few inches to the right-hand side of the 

 Microscope, and falls on a sheet of paper fixed to a 2 ft. drawing 

 board, so that the point of a pencil, which is held in the right 

 hand, is in a convenient position to trace the outline of the 

 projected image. 



I will now proceed to describe the special qualities which some 

 of these prisms have as adapting them for particular purposes. 



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