BemarJcs on Prof. Abhe's Note, etc. % Dr. E. Giltay. 961 



magnifying glass or a Microscope, not to accommodate at all. We 

 must, however, take into consideration, that the image, as it enlarges, 



is at the same time formed at a larger distance ; if in A = 1 — — , 



- has once acquired such a high value that 1 may be neglected, 



then A increases even in the same ratio as ^. And if an image, 

 which I observe grows twice as large, 1 shall see no more detail if 

 at the same time the image is formed at twice the distance, for then 

 the image on the retina, and the number of nerve-ends which are 

 used for its examination, remain the same. 



We clearly see from this, that all depends on the dimensions of 

 the retinal images ; the dimensions of the image formed by eye + 

 magnifying glass will have to be compared with those of the image 

 formed by the eye alone. But where must then the object and the 

 virtual image formed by the lens-system be placed in order that 

 retinal images shall be obtained that are fit for the determination 

 of amplifying power ? 



The determination of the amplifying power of an optical 

 instrument is of course chiefly useful for the comparison of the 

 value of such an instrument when used ivith the eye, with the value 

 of the eye alone. To make this comparison correct, the eye, and 

 eye + optical instrument, should be compared as much as possible 

 under analogous circumstances, which for instance might be realized 

 by comparing them while working as favourably as possible, i. e. 

 giving the largest possible images on the retina. As to seeing by 

 the eye alone, it would be sufficient to bring the object as near as 

 possible to the eye, and then to divide the dimensions of the largest 

 image that might be obtained while using the instrument by the 

 dimensions of the retinal image. This would be a very good mode 

 of determining the absolute value of an amplifying instrument 

 for a single person. But the position of the nearest point which 

 can be accommodated for, differs very much with the person ; and 

 the distance for which during a long time one can easily ac- 

 commodate, will also be subject to much variation. Moreover, the 

 meaning of determining the amplifying power is not so much to 

 know the absolute value of a Microscope for a particular per- 

 son, as to find an expression which is appropriate for the com- 

 parison of Microscopes in general, and which at the same time 

 gives a direct notion of the power of enlarging the images on 

 the retina /or the eye in general; for a simple comparison of the 

 power of microscope-systems their focal distances would suffice. In 

 order to satisfy both conditions, it has been agreed to place the 

 object at a distance conventionally fixed, a distance which is not 

 too great for the retinal images to be near their maximum dimensions, 

 and which is yet large enough for the great majority of eyes to 



