34 Transaction's of tee American Institute. 



metliod of the exact sciences. The doctrines of static electricity only 

 assumed the form of a science after the invention of tlie balance of 

 torsion ; and dynamic electricity is under obligations equally great to 

 the ingenious instrument called the galvanometer. Mineralogy owes 

 the exactness of its determinations to the reflecting goniometer. The 

 brilliant advances of the present century in physical optics liave been 

 aided in a marked degree by the polariscope. Yery recently chemi- 

 cal analysis has found in the spectroscope an admirable auxiliary to 

 the delicacy of its determinations, an auxiliary which has already 

 brought to light four new metals, of whicli the existence had not 

 been previously suspected, and which is busying itself successfully 

 with the elementary constitution of the sun, the fixed stars and the 

 nebulje. In the field of physiology and structural anatomy, it is the 

 miscroscope which has occupied the first rank among the iiistrumen- 

 tal aids to investigation, and which may almost be said to have created 

 the science which it illustrates, in proportion as it has itself advanced 

 toward perfection. 



I will now endeavor to explain, in as brief a manner as possible, 

 and with the help of illustrative diagrams, the construction of this 

 beautiful instrument, and the principles on which its usefulness 

 depends. 



The name microscope defines itself. It is formed from Greek 

 words, signifying to observe the minute, as telescope signifies to dis- 

 cern the distant. Microscopes have been constructed upon the prin- 

 ciples of the reflection of light, as well as upon those of refraction. It 

 is the refracting microscope which is in common use. To understand 

 its construction it is necessary to refer to some elementary properties 

 of light. A inedinin in optics is any space through which light can 

 pass, whether it be void or occupied by matter in a solid, fluid, or 

 gaseous condition. Kays of liglit passing from one medium to another, 

 perpendicularly to the bounding surface, maintain their direction 

 unchanged. If inclined to this surface, they abruptly change their 

 direction, or are refracted. As a rule, the ray will be nearer to the 

 perpendicular in the denser medium than in the rarer. The angle 

 of incidence is that made by the incident ray with the perpendicular, 

 the angle of refraction^ that made by the refracted ray witli the same 

 perpendicular. The laio of refraction is, that the sine of the angle 

 of incidence, divided by the sine of the angle of refraction, gives 

 invariably the same quotient. This quotient or 7'atio is called the 

 index of refraction. Parallel rays transmitted at any inclination, 



