THE MICEOSCOPB 



CHAPTER I 



ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



To be the owner of a well-chosen and admirably equipped 'inicro- 

 scojje, and even to have learnt the general purpose and relations of 

 its parts and appliances, is by no means to be a master of the in- 

 strument, or to be able to employ it to the full point of its 

 efficiency even with moderate magnifying powers. It is an instru- 

 ment of precision, and both on its mechanical and optical sides 

 requires an intelligent understanding of principles before, the best 

 optical results can be invariably obtained. 



We may be in a position, with equal facility, to buy a high-class 

 microscope and a high-class harp ; but the mere possession makes 

 us no more a master of the instrument in the one case than the 

 other. An intelligent understanding and experimental training arc 

 needful to enable the owner to use either instrument. In the ca>r 

 of the microscope, for the great majority of purposes to which it is 

 applied in science, the amount of study and experimental training 

 needed is by comparison incomparably less than in the case of the 

 musical instrument. But the amount required is absolutely essen- 

 tial, the neglect of it being the constant cause of loss of early enthu- 

 siasm and not infrequent total failure. 



In the following pages we propose to treat the elemeiitary 

 principles of the optics of the microscope in a practical manner, not 

 merely laying down dogmatic statements, but endeavouring to show 

 the student how to demonstrate and comprehend the application of 

 each general principle. But in doing this we are bound to re- 

 member a large section of the readers who will employ this treatise, 

 and to so treat the subject that all the examples given, or that may 

 be subsequently required by the ordinary microscopist, may be 

 worked out with no heavier demand upon mathematics than the 

 employment of vulgar fractions- and decimals. 



In like manner, although we shall again and again employ the 

 trigonometrical expression ' sine,' its use will not involve a mathe- 

 matical knowledge of its meaning. The sines of angles may be 



B 



