ZOOLOGY AND BOTANY, iMICROSCOPY, ETC. 319 



dark-coloured 6-in. wing or tail feather so as to leave only a small fan- 

 shaped tip at the end, from h in. to f in. across. With scissors, this 

 is trimmed so as to have a somewhat rounded contoui'. While the 

 right hand is engaged in making the chart, the left hand can flirt this 

 little disc in and out between the objects and the concave mirror and 

 so produce a dark-ground effect as desired. To do this the feather 

 "disc " must be materially smaller than the mirror. 



The charts are nothing more than rude camera lucida drawings of 

 the objects, and with practice can be made with great rapidity. A 

 lot of fiifty nemas mounted under a J -in. round cover-glass can be 

 drawn in two to three minutes with sufficient accuracy to make a very 

 useful chart. (See fig. 1.) Each nema-diagram on the chart has four 

 very distinct properties : (1) position ; (2) form ; (3) size ; (4) orienta- 

 tion. For the most satisfactory work, it is desirable that a certain 

 optimum number of objects exist on the slide. This optimum is deter- 

 mined by the nnmber of them that will appear in a single field of the 

 lens afterward used in searching. Suppose a 16 -mm. objective is nsed 

 as a searching objective, and a 4-mm. for the examination ; then the 

 optimum number of objects under the cover-glass is that nnmber which 

 brings into each field of the 16-mm. objective one to three objects. 



After the chart is made, the short, crooked lines, representing' the 

 nemas, say, are numbered in transversely arranged groups. Each 

 transverse group of the series constitutes a band of nemas running 

 across the mount and having such width as comes fairly well within 

 the scope of a single field of the 16-mm. objective. These imaginary 

 bands are illustrated in fig. 1. It will be seen that there are four such 

 bands. The nemas are numbered more or less consecutively. Pro- 

 ceeding in this manner, on reaching the end of the first band, one 

 numbers the second band, also more or less consecutively, and so on to 

 the end. 



In recording, begin with No. 1, placing it in the field of the 16-mm. 

 objective. It is recognized by its size, form and orientation. Having 

 recorded No. 1 and examined it with the l-mm. objective, a glance at 

 the chart will indicate at what distance, and in what direction. No. 2 

 lies from No. 1. Revolving to the 16-mm. objective and looking 

 through the microscope at Nema No. 1, the slide is moved in the 

 indicated direction until No. 2 is found and recognized. After record- 

 ing No. 2, No. 3 is found in the same way, and so throughout. The 

 novice will be surprised to find how easy it is, with a little practice, to 

 follow the series through without error. 



The drawings should be so made and numbered that the chart and 

 the objects as seen under the microscope will resemble each other. 

 If no care be taken in this respect, the chart may be found to be " left- 

 handed." Securing a " right-handed " chart is merely a matter of 

 properly arranging the paper at the time the chart is drawn. Diagrams 

 should be so made with reference to the printed matter that when it 

 is right side up, the objects as viewed through the microscope will have 

 the same orientations as the diagrams. 



This completes the description of this method, except to explain 

 that in the example illustrated, the numbers encircled are so marked 



z 2 



