equally well over all parts of the field. It must also 

 be noticed here that if the diaphragm is still attached 

 to the instrument and does not swing with the mirror, 

 it may be the means of cutting off light. The largest 

 opening should be employed or, if the cap diaphragm 

 is used, this should be removed. By means of the 

 micrometer screw carry the fine adjustment back and 

 forth beyond the plane of the object and observe 

 closely whether any lines can be distinguished. It is 

 very probable that they will show, but if not, the cause 

 should be determined. It may be that the magnifying 

 power is not sufficiently great to make the lines visible 

 and in this case a higher power eyepiece should be 

 used ; or the cover glass may be more or less than 

 the normal thickness, which would cause a spherical 

 over- or under-correction in the objective. In this 

 case the lines would appear when the outline of the 

 diatom is out of focus, and the structure will be 

 more readily apparent with oblique than with central 

 light. If the above directions have been followed, the 

 lines ought certainly to appear with a moderately good 

 1-5, 1-6 or 1-8 inch objective, but if they are not, the 

 trial should be repeated. Again, be careful to have 

 no obstruction in the course of rays from the mirror 

 to the stage ; have good illumination on the object ; 

 observe well, and keep the instrument in such a posi- 

 tion that the object is not illuminated from any other 

 direction than from the mirror. 



