138 STODDEE^ ON NOBERt's TEST-PLATE^ ETC 



English and American opticians name their objectives 

 (i. e. the lens or lenses placed next the object, that next the 

 eye being the eye-piece) from their magnifying power — 

 thus a i inch objective has the same power as a simple lens 

 of ^ inch focus. Continental European makers generally 

 distinguish their instruments by numbers, the higher num- 

 bers indicating higher powers ; but as each maker has his 

 own system, the actual power of an instrument must be 

 ascertained by trial. Instruments also often differ from their 

 names, and they cannot generally be depended on. The 

 theoretical power of a microscope is measured from an 

 arbitrary standard of ten inches — thus, a one inch is said 

 to magnify ten diameters ; a ^ inch, forty diameters. If the 

 standard is taken at five inches, as it is by some, then 

 the " poAver " is but one half as much. The " power " 

 of the microscope is that of the objective multiplied by 

 that of the eye-piece ; if the objective magnilies ten 

 diameters, and the eye-piece ten, the result is one hundred 

 diameters. 



Angular ap.erture is the angle in the surface of the front 

 lens, at which light will enter the objective — the greater the 

 angular aperture, the more light, and usually the greater 

 resolving power. 



An amplifier is an achromatic combination inserted in the 

 compound body of the instrument to increase the " power " 

 of the objective and eye-piece. 



Immersion lenses have lately attracted great attention, 

 though they were made by Amici many years since. The 

 objective is immersed in water — that is, there is a film of 

 water between the front of the objective and the object, or 

 the thin glass covering it. The effect is a great increase of 

 light, and better definition. 



