206 



KNOWLEDGE. 



June, 1911. 



vanishing point of the figure for the right eye heing 

 separated from that of the figure for the left eye by 

 a distance about etiual to that between the two 



figures ma\- 



be dt 



■d hv the exercise of a ver\- 



httle ingenuity. 



of a tlieorem in 

 ; this drawing in 



Fitu-RH 2. 



eves. (In aduUs this is 

 about J inches, while in 

 boys of about fifteen 

 years of age I have found 

 the average to be about 

 2-5 inches). For all 

 practical purposes the 

 front face of the cube 

 ma\- be represented by 

 a perfect square. The 

 figure also shows the 

 method of obtaining the 

 axis of the cube. It 

 must be remembered 

 that the geometric 

 centre of a face is not 

 necessarily the stereo- 

 scopic centre. If the 

 point of intersectitm of Figike 3. 



the diagonals is taken, 

 no mistake can be 



made. Similarly, to obtain the middle point of an 

 edge, draw a line parallel to an edge which is 

 at right angles to the former through the point of 

 intersection of the diagonals. This will cut the 

 edge at the required point. Thus, in Figure 1. .\ 

 and B are the stereoscopic centres of the edges on 

 which the\- stand. 



I have found it convenient to make the drawing 

 first on a large piece of drawing paper and then 

 prick through the necessary parts on to the paper 

 which is to form the finished slide. This not onlv 

 has the advantage of not showing the construction 

 lines in the stereoscope, but a large number of figures 

 of different subjects ma\' often be pricked off the 

 same drawing. From the cube a vast number of 



Figure 2 represents the figure 

 Euclid. Book XI. By examinin; 

 the stereoscope the figure stands out in relief in a 

 most striking manner, and brings home to the mind 

 of the student the whole meaning of the theorem at 

 once. The construction of these figures bv the 

 student is quite an education in itself, in addition to 

 pro\iding him with figures equal to models in every 

 wa\'. 



Nor does the mathematical student monopolize 

 the benefits to be derived from the use of the 

 stereoscope. Figure 3 shows how an ill-formed 

 crvstal of the octahedral system is derived from 

 the perfect octahedr(.)n : a matter which is by 

 no means clear to e\erv student of chemistry. 

 .\gain. Figure 4 shows the graphical formula of 

 one of the opticallv active forms of tartaric acid. 



I'or ob\ious reasons, if 

 only one of the figures 



figure requires letterinjj 

 should be lettered. 



I ha\e found stereo- 

 scopic drawings of geo- 

 metrical solids very 

 useful in demonstrating 

 to phvsics students the 

 theory of stereoscopic 

 vision. It is perfectly 

 obvious to them that 

 the two drawings of 

 the solid are not alike, 

 and \et, when their 

 images are superimposed 

 b\' the lenses of the 

 stereoscope, they give 

 the idea of relief as 

 plainh" as the solid 

 itself would do. 



COOH 



OH 



COOH 



FiGLKE 4. 



DISCS I'OR SOLAR PK( )| IXTION. 



We give here three more of the Maps for Solar Projection belonging to the series described in 



KnowleDGK" for May. prepared by Mr. John McHarg, which can be used from June 1st tc 



une 2/th. 



