MONOCHROMATIC SUNLIGHT. 459 
tlie best compound objectives of some first class maker ‘should be 
selected. It is a mistake, to suppose, as some have done, that a 
single lens can be substituted for the modern carefully corrected 
compound objective, even if the pure monochromatic light of a 
narrow portion of the solar spectrum as obtained by a prism were 
employed. For the objective always requires to be corrected for 
spherical aberration, and in the case of high powers must be 
provided with a screw collar to modify the distance between the 
posterior combination and the front one in accordance with the 
different thickness of the covering glass of the preparation. Now 
practically the spherical aberration is best corrected by the just 
combination of crown and flint glass, and combinations very 
hearly the same as those employed for white light would still be 
necessary if the objective were made for exclusive use with mono- 
chromatic illumination. 
Under these circumstances I do not recommend the use of 
Monochromatic illumination for low or medium powers except 
When photographs are to be made. It is only as an aid to high 
Power definition that I here commend it. With its aid objectives 
Meapable of resolving certain difficult tests (such as Amphipleura 
pellucida, Grammatophora subtilissima, etc.) with white light, show 
them in a satisfactory manner, and those which even with white 
light are capable of displaying the most difficult tests, exhibit 
them with greater clearness and distinctness. I attribute this 
