1890.] MICROSCOPICAL JOURNAL. 79 



retina, than upon the upper and lower Iialves. Such is the case for the 

 following reasons : 



If one excludes the most peripheral portion of the retina in cases 

 where there is a diti'erence in the shape of the nose root on the temple 

 side of the retina. The right and Itjft halves of the retina of an eye 

 are generally during the whole life atiected 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 contrary, 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 w^anting in the lower 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 ravs from the light which illuminates the H^emometer. 

 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 Hasmometer. This is done b}' mov- 

 ing 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 neighborhood 

 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 again be 

 compared. If there be the slightest doubt, the perfect ecjuality 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 with the 

 conviction that the greatest care and attention has been given. In the 

 use of the Hasmometer, which is so simple that it must be intelligible, 

 the conscience of the observer will in every case tell him of how much 

 confidence he has made himself and his observations worthy. 



But when the observer has been able to reach only a hesitating and 

 unsatisfactory decision, it cannot always be attributed to want ot con- 

 scientious care and attention. 



There are persons w^ho, although they are not exactly red blind, never- 

 theless have a retina very sensitive to long undulations of light, and to 

 such persons the graduation of the Haemometer not only presents a cer- 

 tain difiiculty while it does not allow them to reach a positive conclu- 

 sion satisfactory to themselves, but according to the few experiments ot 

 which they were hitherto capable, it seems that such persons graduate 



