Ch. II] DARK-GROUND ILLUMINATION WITH HIGH POWERS 75 



the object (that is, in passing from the condenser to water the critical 

 angle is 6i°+ Ch. IX), and this obliquity is sufficient for most purposes. 

 Naturally it is easier to clean the water from the condenser and from 

 the under side of the slide than to clean off the homogeneous liquid. 

 For the most satisfactory work with the homogeneous objectives, how- 

 ever, it is better to use the homogeneous connection with the condenser 

 and slide. 



§ 131. Ultramicroscopy. — By this is meant the detection of 

 very small particles by means of dark-ground illumination. The 

 principle is exactly the same as for the dark-ground work already dis- 

 cussed, except that it is carried to the extreme limit by using the 

 brightest light, — sunlight or the electric arc, — and the light rays are 

 practically at right angles to the optic axis of the microscope. From 

 this direction of the light it is evident that none of the rays can enter 

 the microscope with even the widest apertured objectives unless the 

 light is deflected by something in the field. The brilliant light so 

 used renders minute particles luminous something as sunlight entering 

 a small hole in a darkened room renders particles of dust luminous. 

 The greater the angle between the optic axis of the microscope and the 

 light reaching the object, the smaller can be the object which will 

 be revealed. As this method of lighting rendered particles luminous 

 and therefore visible that were invisible with the microscope as ordi- 

 narily used, the use of the microscope with this lighting has come to 

 be called Ultramicroscopy. 



Ultramicroscopy required very special apparatus to get successful 

 results, and is not at present much used in ordinary biological study 

 and investigations. The matter has been ably treated in Dr. Chamot's 

 work on Elementary Chemical Microscopy, Ch. IV, and the reader is 

 referred to that treatise for a full and satisfactory account. 



§ 132. Gordon's method for dark-ground illumination with high 

 powers. — This method as given by Wright has the advantage of 

 utilizing the full aperture of the objective and of requiring a very small 

 amount of inexpensive apparatus in addition to the regular micro- 

 scopic outfit in the hands of every worker. The method is as follows: 



The object is lighted by a solid cone of light from the condenser as 

 usual, but the aperture of the condenser must only fill the middle 



