110 Modern Microscopy 



circuit. The lamp should have a frosted bulb, and have 

 a diaphragm, preferably of the iris form, placed beneath 

 it. If the opening of this diaphragm 'be treated as the 

 source of light, very good results can be secured. 



Incandescent Gas Lamps. A very brilliant and efficient 

 illumination can be obtained from an incandescent burner 

 on a table-stand of suitable height properly shaded. The 

 reticulations of the mantle would render it objectionable, 

 but this can be obviated by placing an iris diaphragm a 

 short distance from the light in the same manner as is 

 described for the electric lamp above, or a narrow slit 

 would answer the purpose equally as well ; the diaphragm 

 or slit would then be focused by the sub-stage condenser, 

 and treated as the source of light. 



The Revolving Nosepiece. 



Time-saving arrangements will often be found useful in 

 work, and the nosepiece is one of these. This is a piece of 

 apparatus which is screwed into the 

 fixed nosepiece of the microscope, 

 and is made to carry either two, 

 three, or four objectives, termed 



respectively the double, triple, or 



FIG 42 THE DUSTPROOF 



NOSEPIECE FOR THREE quadruple nosepiece. Each of the 



OBJECTIVES. objectives can in turn be rotated 



into the optical axis, thus saving 



the necessity of unscrewing an objective and screwing 

 another on in order to get ' a variation of power. In 

 hospitals, laboratories, etc., it is usual to have one of these 

 fitted to nearly every instrument. A new pattern which is 

 described as dust-proof has found special favour, rendering 

 unnecessary the removal of objectives after use. One of 

 these is illustrated (Fig. 42). They are to be had made 

 of an aluminium alloy which is extremely light, reducing 

 the strain on the body tube. Any microscope having the 

 universal size of thread for objectives will carry a revolving 



