296 TECHNICAL MICROSCOPY. 



a + or direction from the real size. If we are examining 

 tabular crystals which are to be regarded as truncated clinorhom- 

 bical columns (Fig. 165), the equal breadth of the column-surfaces 

 (shaded in the figure) is a proof that the terminal face is 

 not inclined either to the right or left. In this position, 

 therefore, the angles a and b cannot, at any rate, appear 

 smaller than they really are, since an inclination in the 

 direction of a to b would in all cases magnify them. 

 Similar data are also furnished by many other crystal- 

 forms, and the observer must discover them in every given 



FIG 165 case anc ^ ^ a ^ e no ^ e ^ them. 0^ what value are gonio- 

 meters which exhibit the arcs of the revolutions accurately 

 up to one minute if the errors arising from the object amount to 

 1-2 degrees ? No one would expect that such errors in small 

 octahedrons, rhombohedrons, &c., could be easily avoided. 

 Repeated experience has .shown us that a skilful observer is 

 required to effect the accurate determination up to ^ degree of an 

 angle in crystals such as are commonly found in vegetable cells, 

 or as they arise after the addition of reagents to the fluid under 

 observation. The goniometer might in such circumstances be 

 regarded as perfect. 



V. 



THE DRAWING OF MICROSCOPIC OBJECTS. 



THOSE who are much occupied with microscopical investigations 

 ought to be able to make drawings of what they have observed 

 agreeing in all essential points with the mental impressions they 

 have received. We say with the impressions received, not with the 

 object itself; for a truth to nature surpassing this relative one 

 is not imaginable. We cannot reasonably demand that the hand 

 of the draughtsman should figure correctly what the eye of the 

 observer has seen incorrectly. The acquirements which are usually 

 expected of the draughtsman correct conception, understanding 

 the nature of the object, truth to nature without subjectivity, &c. 

 must therefore be strictly demanded of the observer ; the draughts- 

 man's part is merely to call up in the mind of another the impres- 



