ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 279 



by the convergence of the visual axes. It is quite different in drawing 

 with the camera. If it is desired to avoid the inconvenient elevation 

 of the drawing-surface, accommodation is necessary for the distance 

 of this surface, and such accommodation is not assisted by a converg- 

 ence of the visual axes. Many persons are therefore not able, with- 

 out great fatigue, to accommodate their eye sufficiently to see the 

 pencil clearly. This consideration suggests the remedy. If the 

 observer is emmetropic (or normal-sighted), it is only necessary to 

 insert a lens in the path of the rays proceeding from the paper to the 

 eye, having a focus equal to the distance from the paper to the lens. 

 The rays from the drawing-point are then changed into parallel 

 pencils, and the eye sees the point with perfect distinctness, although 

 accommodation is quiescent. If, however, the observer is ametropia 

 (short-sighted or far-sighted), then a lens must be interposed which 

 allows the rays proceeding from the paper to be directed after their 

 exit from the lens to a surface situated at the distance of the punctum 

 remotum. 



The mode of choosing the appropriate spectacle glasses is then 

 given in some detail, and these concluding remarks : " Man, as is 

 well known, is in a high degree a slave of habit. When we begin to 

 use the Microscope, it is difficult, on account of the reversal of the 

 movements, to guide the object. If we are, however, once accustomed 

 to it, and work occasionally with a dissecting lens, then the difficulty 

 presents itself of effecting the movements which we formerly did a 

 hundred times daily. The reversal of the movements has associated 

 itself with the act of using the Microscope. It is the same also with 

 the accommodation of the eye. When we begin to work with the 

 Microscope it is tiring, probably for the most part on account of the 

 effort of accommodation. One soon learns to relax the accommodation- 

 muscles while working with the Microscope. When we have become 

 adepts at this, and wish to draw by means of the camera, then the 

 requisite accommodation again at first gives trouble. If finally an 

 apparatus is fixed to the camera, by which no accommodation is needed, 

 then it may happen that we cannot at once adapt ourselves to it, 

 because in using the camera we had got accustomed to the exertion of 

 accommodation. We soon, however, learn all this, only we must not 

 too hastily consider a lens found by calculation as too strong, for 

 this may occur through a false computation of the punctum remotum, 

 or by not relaxing the accommodation in using the Microscope." 



Hilgendorf 's " Apparatus for Microscopical Geometrical Draw- 

 ings-"* — Dr. F. Hilgendorf describes an apparatus which is essentially 

 a pantograph with the usual four arms, but in which the tracing point 

 is replaced by a sight-vane. This is about 20 cm. above the arm, on 

 which is a lens magnifying three to four times, and having crossed 

 threads on its upper surface. If the outlines of the object are followed 

 with the sight-vane, the pencil at the end of a prolongation of the 

 opposite arm will produce an enlarged drawing of the outlines on the 

 paper beneath it. 



* SB. Gesell. Naturf. Freunde zu Berlin, 1882, pp. 58-60 (1 fig.). Zeitschr. 

 f. Instrumentenk., ii. (1882) pp. 459-60 (1 fig.). 



