36 THEORY OF THE MICROSCOPE. 



is, of course, not to be regarded as the object, but as the image 

 which the objective would form alone (without the aid of the 

 field-lens) ; it must be regarded as a virtual image, since it lies 

 behind the first refracting surface of the system. Its position is 

 determined by the final virtual image of the eye-lens, which for 

 each observer is situated at the distance of distinct vision from 

 the eye ; it must, therefore, be different for different eyes, since 

 this distance is variable. In order to have a definite idea on this 

 point, let us suppose that the virtual image lies in a plane 

 situated 200 mm. from the last surface of the eye-piece ; the 

 posterior focus p* must accordingly be taken as equal to 

 144f mm. ; whence the conjugate value p = 36*041 mm. 



The real image of the objective would consequently have to be 

 formed at a distance of 12*041 mm. from the last surface of the 

 eye-piece, in order to be seen by the eye placed at the given 

 distance of 200 mm. The amplification m, which is produced 

 by the eye-piece under these circumstances, is found by the 

 formula 



m = -r- = 4-01 . 

 f - P 



If, then, the microscope-tube is of such a length that the posterior 

 focus of the objective, taken above at 200 mm., corresponds to the 

 position of the objective-image relatively to the eye-piece as just 

 described, then the total magnifying power of the Microscope is 

 evidently m times that of the objective as above found. For any 

 other length of the microscope-tube, however, the posterior focus 

 /)* of the objective must first be calculated, and the magnifying 

 power then reduced in accordance therewith. If, for instance, the 

 last surface of the eye-piece is at a distance of 200 mm. from the 

 first surface of the objective, and the second principal point of the 

 latter at a distance of 2*4011 from the anterior surface, we obtain 

 for p* the value 200 - (12-041 + 24011) = 185-558, and, from 



* 



this, an objective amplification in = 1 = 81*584, which 



latter number multiplied by 4*01 gives the total magnifying power 

 of the Microscope. 



The magnifying power of a Microscope is not a definite amount 

 which can be fixed once for all. It is dependent upon the condition 

 of the eye of the observer, and cannot be considered a constant 



