Eemarks on Prof. Ahhes Note, &c. By Dr. E. GiUmj. 963 



The linear amplification as it is here defined is exclusively a 

 measure for the amplifying power of the system ; and the com- 

 parison of the difierent linear amplification values of different 

 systems gives a direct comparison between the value of the difierent 

 systems with regard to their power of enlarging the images on the 

 retina. 



My definition of the " linear amplification " is not new ; it is 

 in complete accordance with the meaning which is commonly 

 attached to the term ; I only tried to treat the subject more amply 

 than is generally the case in physical or Microscopical text-books, 

 and to give a fuller account of the meaning that must be attached, 

 according to my ojnnion, to the " distance of distinct vision " than 

 has hitherto been done as far as I know of.* 



Very often false notions have been attached to the amplification 

 values of optical systems. I do not believe, however, that these 

 false notions originated in any unfitness of the usual definition of 

 amplification, but I think that the chief cause lay in the awkward 

 ideas which are still too commonly found on even the simplest facts 

 of theoretical microscopy. 



These erroneous ideas with regard to the meaning of " hnear 

 amplification" are chiefly the following: — 



1. The " Hnear ampHfication " is taken as identical with the 

 actual amplification that the virtual image, which is observed 

 when using the instrument, undergoes in each particular case. 



2. These two kinds of amplification, the properly so-called 

 linear and the actual amplification for a particular case, are again 

 confused with the amplification which the image, which the 

 observer " sees " before him, has undergone, and which is of a 

 purely subjective nature. 



Both points still need perhaps some explanation. 



1. When any object is observed through an optical system, as 

 for instance a Microscope, this instrument so alters the course of 

 the rays emanating from the difierent points of the object, that 

 an enlarged image is substituted for the latter. The place of this 

 (virtual) image depends upon the relative position of the optical 

 system and the object, and the latter is so regulated by the ob- 

 server that the image in question falls at a distance which is in 

 accordance with the refractive condition of the eye, in order that 

 a well-defined image may be formed on the retina. As the refrac- 

 tive conditions of difierent eyes vary very much, the distance of the 

 image is also very difierent ; it may be a few inches, and it may be 

 several yards. And as the quotient of the diameter of that image 



* I must here add tliat the reasoning wliich I used to oxplaiii the term 

 "distance of distinct vision," was not followed in the same wav (as far as I know, 

 at leaiit) when the tf-rm was firht brought into use. I only used the above line of 

 argument l)ecause it s<erned to me the fittest way to make the matter quite eh ar. 



