ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 317 



College, the instrument has shown itself to be satisfactory as regards 

 the accuracy of construction of circle and verniers, and the optical 

 definition is good ; while the design is generally convenient in use. It 

 has been incidentally found that fixed magnifiers for reading the lower 

 circle are desirable, but as the dividing in the present instrument is 

 very clear and easy to read these magnifiers could have a longer focus 

 than usual, thus preventing the usual shadowing of the circle by the 

 head of the observer. No doubt such lenses could be provided very 

 easily by the makers. 



Owing to this clearness of dividing of the circles, also to the 

 simplicity and great strength of the design, the instrument is likely to" 

 be very useful for teaching purposes. It should also find a sphere of 

 usefulness in the testing laboratory, especially if a micrometer fine 

 adjustment similar to that of the Pulfrich refractometer and to those of 

 certain old Steinheil spectrometers were given to the telescope. Results 

 fo]' the dispersions of prisms could then be obtained with an accuracy 

 comparable with that of the value for absolute refractive index — an 

 impossibility when the ordinary vernier measurements are alone possible. 



If a more elaborate instrument were desired, micrometer microscopes 

 for the main circle could doubtless be provided, and, as was previously, 

 indicated, a larger telescope and collimator could easily be carried by 

 the same supports. 



This is surely a praiseworthy principle of design that the workman- 

 ship of even the simpler types of instrument should be the very best. 

 Any improvements or additions later found to be necessary will not be 

 wasted on the original. The illustration shows one of the earlier trial 

 models. Verniers are now supplied to the table circle in addition to 

 the main circle. (The illustration on p. 316 is inserted by the courtesy 

 of the editor of The Transactions of the Optical Society-) 



B. Technique. 



Suggestions for Methods and Apparatus, by N". A. Cobb, United 

 States Department of Agriculture '"' : — 



Systematically Examining Large Series of Microscopical Objects. — • 

 There are various methods of recording the position and character of 

 each member of a large series of objects mounted on a microscope slide. 

 One of the commonest methods involves the use of a recording, 

 mechanical stage. Each object on the slide receives a record-number 

 consisting of two separate readings from scales engraved on the 

 mechanical stage. The following method, however, is successful with- 

 out a mechanical stage or finder of any sort, and is characterized by 

 simplicity and expedition. It may be called the method of charting. 



The method consists in making a camera lucida drawing or chart, 

 at low magnification, of all the objects of which it is desired to make 

 record. The chart is diagrammatic ; each object is represented on the 

 chart by a simple, characteristic diagram, and the diagrams are then 



* This article was contributed to thg Transactions of the American Micro- 

 scopical Society, October, 1920. 



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