GENERAL NOTES 139 



embrace but nine of the one-tenth divisions 

 on scale, then the amplification is shown to 

 be only ninety diameters, as five would give 

 fifty, etc. Intermediate enlargements between 

 the tenths must be carefully calculated. This 

 method is, perhaps, not always accurate to 

 a single diameter, but is sufficiently so for 

 all practical purposes. If the enlargement is 

 more than 100 diameters it will be well to 

 measure the distance between two rulings 

 when using the T^O scale. With high pow- 

 ers it will be necessary to employ the ToVo 

 rulings, ten spaces of which enlarged to 

 one inch on the focusing screen are equal 

 to one thousand diameters. It will be found 

 very useful to ascertain the exact camera 

 extension necessary to give, say 100, 250, 

 500 and 1,000 diameters with combinations 

 of the various objectives and eye-pieces; 

 making records of the same for future use. 

 Then, if it be desired to photo-micrograph 

 an object at any particular number of diame- 

 ters, by referring to note-book, the combina- 

 tion of objective, ocular and camera exten- 

 sion, which will give this amplification, may 

 be found at once, thereby saving much time 

 and the necessity of making fresh measure- 

 ments. Tissue work is usually done under 



