ABBE, AND MICROSCOPE THEORY IN GERMANY. 165 



of the term " Magnification Power," which is measured by the 

 reciprocal of the focal length.* 



By this he wished to combat the idea, particularly prevalent 

 among microscopists, that the efficiency of the microscope is 

 dependent on the accommodation of the eye of the observer. On 

 account of the somewhat abstract conception of the idea of 'power 

 of magnification, this expression, which is without doubt much 

 more correct, has been almost entirely ignored, and the tables of 

 magnification in the catalogues of optical works still contain 

 linear magnifications for vision at 250 mm., although experience 

 shows that this sometimes gives rise to the most curious 

 notions. 



Of the rest of the first volume of the collection of treatises 

 under consideration, we will only mention VII., "On Micro- 

 metric Measurement by Means of Optical Images," and VIII., 

 "On the Counting of Blood Corpuscles." In the first, the 

 importance of the telocentric ray-path, for making the measure- 

 ments independent of the position of the focussing plane, is 

 shown ; and, in the second, Abbe, on the basis of the calculus of 

 probabilities, discusses in a thorough manner the question of the 

 trustworthiness of the countings by means of the so-called count- 

 ing chambers, which are ruled in squares, and gives lucid instruc- 

 tions for the judging of probable faults not connected with the 

 faults of the apparatus. The critical employment of these 

 methods of counting, which have become so important for medical 

 diagnosis, was essentially advanced by this. 



By the references given in the above review, the epoch-marking 

 importance for microscopic observation of Abbe's works will be 

 patent to every reader. On that account it is hoped the unusual 

 length of this review will be excused, as it seemed particularly 

 desirable to give more than a cursory reference on the appearance 

 of this collection. 



There was also another reason in favour of a detailed review. 

 Scientific microscopy is no longer, as was the case a few decades 

 ago, merely the servant of the biological sciences ; the researches 

 of Abbe have given it a position of its own in the world of science. 

 The extent to which this science is taught at universities is, 

 however, pitifully small. Instruction in the use of the microscope, 



* In ophthalmology a similar expression had been introduced at an earlier 

 period for the power of spectacle lenses (Dioptric). 



