PEEFACE TO THE SEVENTH EDITION vii 



has, even since the last edition of this book was published, been so 

 marked as to produce a revolution in the instrument itself and in its 

 application. The new dispensation was dimly indicated in the last 

 edition ; but it has effected so radical a change in all that apper- 

 tains to Microscopy that a thorough revision of the treatment of 

 this treatise was required. The great principles involved in the 

 use of the new objectives and the interpretation of the images pre- 

 sented by their means, are distinct and unique ; and unless these be 

 clearly understood the intelligent use of the finest optical appliances 

 now produced by mathematical and practical optics cannot be 

 brought about. They have not rendered the use of the instrument 

 more difficult they have rather simplified its employment, provided 

 the operator understand the general nature and conditions on 

 which his Microscope should be used. If the modern Microscope be, 

 as a mechanical instrument with its accompanying optical apparatus, 

 as good as it can be, a critical image a picture of the object having 

 the most delicately beautiful character is attainable with 'low 

 powers' and 'high powers' alike. Microscopists are no longer 

 divisible into those who work with 'high powers' and those who 

 work with ' low powers.' ISTo one can work properly with either 

 if he does not understand the theory of their construction and the 

 principles upon which to interpret the results of their employment. 

 If he is familial- with these the employment of any range of magni- 

 fying power is simply a question of care, experiment, and practice ; 

 the principles applicable to the one are involved in the other. Thus, 

 for example, a proper understanding of the nature and mode of 

 optical action of a ' sub-stage condenser ' is as essential for the very 

 finest results in the use of a 1-inch object-glass as in the use of a 

 2 mm. with N.A. 1'40 or the 2'5 mm. with 1ST. A. 1'60, while it 

 gives advantages not otherwise realisable if the right class of con- 

 denser used in the right way be employed with the older ^th inch 

 or ^th inch achromatic objectives, and especially the ^th inch 

 and Jyth inch objectives of Powell and Lealand, of N.A. 1'50. 

 Without comparing the value of the respective lenses, the best 

 possible results in every case will depend upon a knowledge of the 

 nature of the instrument, the quality of the condenser required by 

 it, and its employment upon right principles. 



This is but one instance out of the whole range of manipulation 

 in Microscopy to which the same principles apply. 



In its present form, therefore, a treatise of this sort, preserving 

 the original idea of its Author and ranging from the theory and 

 construction of the Microscope and its essential apparatus, embracing 

 a discussion of all their principal forms, and the right use of each, and 

 passing to a consideration of the best methods of preparation and 



