194 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



light is required to be reflected at an exact right angle. It is of the 

 greatest service when the microscope is of necessity used in a rigidly 

 upright position. 



If it be used for angles other than right angles, there will be 

 refraction as well as reflexion ; and as the necessary decomposition 

 of the light into a spectrum will accompany the refraction, care must 

 be exercised to see that the rays emerging from the prism are at 

 right angles to those incident to it, and that the areas of the square 

 faces of the prism are sufficiently large to have inscribed within 

 them a circle equal to the back lens of any condenser used. 



Some employ what has been known as a ' white cloud illuminator ',' 

 that is, a disc of plaster of Paris, or opal glass with a polished 

 surface . But a disc of finely ground glass dropped into the diaphragm- 

 holder of the condenser will give a precisely similar result. 



Mr. A. Michael has, however, pointed out the curious fact that an 

 opalescent mirror becomes an inexpensive and excellent substitute 

 for a polarising prism. 



Typical Modern Microscopes. We are now in a position to care- 

 fully inspect the characteristics of the chief forms of microscope 

 which the modern manufacturers of England, the Continent, and 

 America offer to the microscopist. 



We confine ourselves to the chief models, indicating more or less 

 suggestively their merits or defects. We neither discuss all the 

 instruments of any maker nor in every case even one instrument of 

 some makers. This would involve simple repetition in the main 

 features. The reader can compare for himself the microscope of any 

 given maker from whose catalogue he proposes to select, and can 

 discover by comparison its incidence or otherwise with the type 

 given here to which it corresponds. 



Beginning with the highest types we place first on the list Powell 

 and Lealand's No. I. This instrument may claim a seniority over 

 all the foremost instruments, because for nearly fifty years it has 

 practically remained the same. All its principal features were 

 brought to their present perfection nearly fifty years ago, while all 

 other microscopes during this period have been redesigned and 

 materially altered over and over again. This is no small commenda- 

 tion, for during that period, as the reader so well knows, the aper- 

 tures of objectives have been enormously enlarged, and with this 

 has come a great increase of focal sensibility. As a result the 

 majority of the microscopes of forty years ago are absolutely useless 

 for the objectives of to-day, but the focussing and stage movements 

 of Powell and Lealand's microscope still hold the first place. 



Fig. 157 represents the instrument in its monocular form. The 

 foot of the stand is a tripod in one casting ; it has an extended base 

 of 7 X 9 inches, forming at once the steadiest and the lightest foot 

 of any existing microscope. The feet are plugged with cork, and 

 when the body is in a horizontal position the optic axis is (as it 

 should be) 10 inches from the table. 



The coarse adjustment is effected by a bar, consisting of a mas- 

 sive gun-metal truncated prism in form, which bears only on a 

 narrow part at the angles. It extends sufficiently to focus a 



