184 NOTES AND MEMORANDA. 



before, lines or beading were beautifully displayed, he sees a blank. 

 He may spend long hours in trying every trick of illumination, mode- 

 rating his light, varying its obliquity by altering the angle of his 

 mirror, focussing and re-focussing the condenser, altering the adjust- 

 ment of his objective ; and at last, when his patience is well-nigh ex- 

 hausted, the desired result is obtained, the delicate markings start 

 suddenly into view, and he possesses the consciousness that, under his 

 hands, mirror, condenser, and objective are now doing their best. Has 

 this time been wasted ? I think not. He will carry the knowledge 

 obtained in the struggle, and apply it in the broad field of real work 

 that lies before him on every side. Should he turn his attention to 

 the development of minute life, organs are seen in living transparent 

 bodies where before he saw nothing ; should he be a pathologist, 

 tissues appear full of structure which before in his inexperienced 

 hands seemed homogeneous, minute nerve-fibres become visible where 

 before they were unsuspected. I do not think I am exaggerating in 

 saying what I have ; I have felt the benefit conveyed in an education 

 of this kind, and I could recommend nothing better for a beginner 

 than a year's constant study of all the species of Diatomacece at his 

 command. When he is fully convinced that he sees all in them that 

 his optical means will allow, he is far better fitted to commence real 

 work than he ever could have been without this preliminary training. 

 Only, let us not mistake ; our work, though commencing on diatoms, 

 should not end there ; let their delicate lines be the means of familiar- 

 izing ourselves with the optical capabilities of the noble instruments 

 at our disposal, and the questions I have quoted will be duly answered 

 — the time spent will not be in vain. 



To the same effect are some remarks made by Mr. J. Mayall, 

 jun., in an address on " Immersion Illuminators," recently delivered 

 before the Brighton and Sussex Natural History Society : — " Practice 

 with diatoms should be regarded as the gymnastics of the Microscope. 

 To ignore this practice is voluntarily to paralyze our possible skill, 

 which cannot be done with impunity, as is proved by the immense 

 mass of old results that are constantly being discarded to make way 

 for interpretations based on more perfect instrumental and manipula- 

 tive means. The improvements in the Microscope are almost wholly 

 due to the criticisms of amateurs skilled in the exhibition of test 

 objects." 



Measurement of the Amplification of Optical Instruments.— 

 The following was communicated by M. Govi to the French Academy, 

 and, being printed in their ' Proceedings,'* we have thought that a 

 translation might properly find a place here. Those who may not 

 altogether agree with the views expressed, may still find some interest 

 in the fact of the paper having been accepted by the Academy : — 



By amplification is meant the relationship of size between the 

 image and the object. The idea of size, obtained by looking at an 

 image, without actually measuring it, is not in any way precise. 



If optical instruments only gave real images, their magnifying 



* ' Comptcs Rcudus,' vol. Ixxxvii. (1878) p. 726. 



