18 CONCERNING THE RULES AND APPLIANCES 



the temple side of the retina. The right and left halves of the 

 retina of an eye are generally during the whole life affected by 

 light and shade to the same degree. In other words, they are 

 blended in the same degree, and consequently are equally sensitive 

 to light. The upper and lower halves of the retina, on the con- 

 trary, are subject to the effect of light in essentially different 

 degrees, in that the picture of the firmament, which in general 

 represents by far the brightest part of the range of vision, is 

 always wanting in the lov/er half of the retina. Thereby it is kept 

 more nearly blinded ; that is, less sensitive to light. 



The observer must also take care that the observing eye is not 

 affected by rays from the light which illuminates the Haemometer. 

 For in this case, in consequence of the lights penetrating the 

 tissues (tunice) of the eye, a similar inequality between the two 

 sides or retina halves may result, such as we have just found in the 

 halves of the retina lying one over the other. 



The real work now is to focus the Haemometer. This is done 

 by moving the glass wedge by means of a large hand piece back 

 of the column until the difference in the appearance of both 

 halves of the comparing vessel has disappeared. This movement, 

 as soon as the neighbourhood of the real graduating point is 

 reached, should be backward, and by short, quick strokes, rather 

 than by a constant slow motion. 



The paths of the wedge as it is shoved from one side to the 

 other over the proper point should be gradually shortened; in this 

 way the distance traversed is lessened while the decision vacillates, 

 until one has at last decided upon the graduation. 



As it is advisable to look often rather than long into the 

 instrument, so also when the graduation point is supposed to be 

 determined, the eye should be averted for a short time either by 

 closing it or by looking at some dark surface, and then both halves 

 of the vessel should be again compared. If there be the slightest 

 doubt, the perfect equality of both halves should again be sought 

 by short backward movements of the wedge, until at length 

 further observation can detect no change in the decision either as 

 to the purport or as to the exactness. 



The sense of perfect exactness and unconditional correctness 

 of the decision will be experienced in each case at the same time 



