294 EDWARD M. NELSON ON 



be likened to the musical gamut, for musical notes are higher and 

 lower than one another, and so are objectives in power. Music 

 is arranged in keys, and so may be objectives — for example, we 

 have — 



Key A. 4, 2, 1, 1/2, 1/4, 1/8, 1/16 in. 

 Key B. 3, 1^-, 2/3, 1/3, 1/6, 1/12 in. 



Besides which there are some sharps and flats, interlopers, 3/4, 

 4/10, 1/5, 1/7, 1/9, 1/14 in. 



Eyepieces also can be arranged in keys thus : 



Key 1. Achromats and Complanats, x 5, x 10, x 20, x 40. 

 Key 2. Achromats and Complanats, x 7|, x 15, x 30. 

 Key 3. Compensating, x 8, x 12, x 18, x 27. 



Note, the above is not mere rhetorical embroidery, but a part of a 

 microscopist's training is to have in his mind some such classifica- 

 tion of objectives and eyepieces. It is quite obvious that at the 

 present time, whatever may have happened in pre-war days, none 

 but a millionaire would ever think of buying all the lenses of both 

 keys ; it therefore becomes a question of great importance which 

 are the best lenses to have. But with regard to our new members, 

 we surely should hold out to them a helping hand, and not leave 

 them, as we have done hitherto, to find this out for themselves — 

 in other words, to buy their own experience at considerable loss. 



At the outset, obviously, the first point to be decided is the 

 choice of key. Is it to be A or B ? I am not going to decide this 

 point for the tyro, but will merely remark that when once the key 

 is chosen, it is bad policy to depart from it and play with lenses in 

 another key. Passing by, then, the debatable points, a few hints 

 from an old microscopist may be helpful to beginners. 



Here is an epigram to start with : A wise microscopist has a 

 battery rich in low powers ; when beginners take up microscopy 

 they, almost without exception, rush after high powers. This is 

 not merely an error, it is sheer recklessness. A good 2 in. and a 

 7| eyepiece is a most important combination, for it will open out 

 the worlds of entomology, botany, pond life, f orams, the sea life of 

 rock-pools, geological sections, etc. There is no other lens that 

 can do half as much or be so often generally useful. 



If a beginner says, I am not going to study any particular sub- 



