SUPPLEMENT. 385 



rays are per force excluded. The objective will not receive 

 them, and there can be no possible doubt but that such work is 

 being done with centrally disposed light, such as the observer 

 desires ; there is no special manipulation or management neces- 

 sary to secure this, for the objective will admit centrally dis- 

 posed rays, and none other. 



But in the handling of a wide-angled lens the conditions are 

 essentially changed, and this change of condition involves in 

 turn a change of handling, and of management. 



Hence it occurs, that in using a wide-angled glass by central 

 or centrally disposed light, some arrangement must be provided 

 for the purpose of shutting out the lateral rays ; these the glass 

 will admit, and in default of the provision referred to, will steal 

 in, cause interference, and defeat both observer and objective. 

 To shut out these lateral pencils would seem eminently the bus- 

 iness of the usual diaphragm box. 



This provoking little piece of apparatus, this diaphragm box, 

 is supplied with almost every stand in use, and is as faulty in 

 operation as human ingenuity in construction could devise ; in 

 general these boxes are furnished with a shutter pierced with 

 openings of various sizes, and placed from one-half to one inch 

 below the object-carrier of the stage. Now, suppose we are 

 using a wide-angled lens, and being desirous of central light 

 only, we attempt the use of the smallest opening of the shutter. 

 What now is the result ? 



It is as follows : the pencils of light enter the small aperture, 

 emerge and diverge within, and fill the box with simply light of 

 low intensity. It has now become a washed-out, wire-drawn 

 light, lacking force, and, I may say, velocity ; the objective now 

 in turn receives this much- abused illumination, not only the 

 central pencils thereof, but lateral ones also ; and the working 

 angle used will be determined by the depth and diameter of the 

 diaphragm box. 



A proper diaphragm for general use not diaphragm box, for 

 the less of the box the better is a plate pierced with a central 

 opening of about the size of a large needle, and so mounted as 

 to approach the iower edge of the mount closely as possible 

 without suffering actual contact. To construct this little piece 



25 Microscopy. 



