198 HOW TO SEE WITH THE MICROSCOPE. 



Not long since I was honored with a call from a gen- 

 tleman who had put himself to some little trouble in 

 visiting Cleveland. Said he: "I have been for some 

 time desirous of visiting you ; I have read all your ar- 

 ticles in the journals, and have been permitted to read 

 your letters to our mutual friend, - .1 want 



to know something more about the " balsam angle " 

 business; ditto, about the " 180"; I confess these ex- 

 treme apertures seem to me "impossible"; and then, 

 again, we have high authority that the true aperture of 

 any object-glass cannot exceed 120 and the assertion is 

 fortified with reasoning that I cannot well dodge. I 

 have brought with me an excellent glass purporting to 

 have corrected angle up to 140, and would like to have 

 some comparisons made with your own. I am after the 

 facts, and have no personal bias in any direction," etc., 

 etc. He also went on to state that Mr. Wenham had 

 expressly asserted that direct light could be obtained 

 with the " reflex." At his suggestion I showed him 

 the No. 18 of the balsam Moller probbe plate illumina- 

 ted with the genuine " reflex." The field was brilliantly 

 lighted with just enough of the blue in to take off' the 

 intensity of the glare. The shell appeared without sen- 

 sible distortion, edges sharp and clean, and with a full 

 stand of lines from end to end. Revolving the stage so 

 as to place the saxonica in a diagonal position, we had 

 little difficulty in obtaining simultaneous views of both 

 transverse and longitudinal striae, thus cutting the valve 

 into checks or squares. The same little hand-lamp was 

 used, and we had nice shows with eye-pieces up to the 

 one-fourth inch. 



