36 THE AMERICAN MONTHLY [Mar. 



test its registering action for particular objects in a 

 mount of a square inch of area or less. Placing the glass 

 stage plate in position on the metal stage, a mount of 

 strewn diatoms is placed in the sliding carrier. A mo- 

 mentary examination may show some single object of in- 

 terest and this is brought to the center of the field of view. 

 Allow the object to remain there for a moment, while 

 with a pen you put a dot of black ink exactly over the 

 guage dot traced on the metal stage plate. Repeat this 

 for any other objects that may be noted to the limit of 

 five or more. If the mount is now set aside the ink dots 

 register, on the slide, the several fields of the objects so 

 noted and may be found at any subsequent time. This 

 is the widest application of the combination of the two 

 straight lines and is a means of locating the intersection 

 of two co-ordinate lines at the point of their intersection 

 from the fixed center of theobjective in use. If one ignore 

 the guage dot, the position of any object may be closely 

 registeredby the aid of the single line or directrix marked 

 on the glass stage plate. Any object noted in the mount 

 is brought to the center of the field, and allowed to rest 

 there a moment. An ink dot is made on the slide at any 

 point along, but immediately over the line. If after do- 

 ing this the slide is run out of the field of view, on again 

 placing the ink dot above the reference line, the whole 

 glass plate is slid carefully through the field when the 

 object is likely to be found by a slight oscilation of the 

 slide while passing through the field. Since the real 

 field under a power of 500 diameters may be 2-lOOths of 

 an inch, the shifting amplitude of the slide carrier should 

 be very minute. In the vermicular shifting of a slide by 

 hand in the usual way in a field of 500-600 diameters 

 the act might consume ten minutes before the desired ob- 

 ject could be found. 



To explore slides without missing a particle of their 

 contents, it can be used this way : Any histological or 



