110 



PHOTO-MICROGRAPHY 



SECTION I. — (i) The different methods of illummating an object placed on 

 the microscope, of which there are nine varieties. 



1. Direct Light. — This is the usual form when photographing ; where the 

 mirror is removed and the microscope illuminated immediately by the illuminant, 

 whether with or without the auxihary condeuser, Fig. 24a. page 54. In employing 

 this form, the jet being lighted, the objective and eye -piece m situ, the object is 

 focussed. The substage condenser is then centred by means of its adjusting screws 

 in a manner already explained page 103, and the auxiliary condenser, if employed, 

 and the limelight are also placed central with the axis of the microscope in the 

 manner described on page 105 as shown in Fig. 24b, page 54 until the field is equally 

 illuminated. 



This may be no easy matter. If the lime be placed too near the jet, it burns 

 away with such rapidity that small pieces are apt-to fly ofi", which makes the field 

 brighter in some parts than others. If too far off", the area of the illuminant is too 

 large, and with very high power work, definition may possibly suffer. Direct experi- 

 ment will soon teach the operator the best position of the lime : we usually find for 

 most work, with the jet by Beard, it is about one-eighth of an inch from the mouth 

 of jet. " Centreing of the lime " as it is called, is made in our arrangement most 

 easily and eflectually by the two screws in the stand already described page 5. It is 

 usually done before screwing up the auxiliary condenser in its place. This effected, the 

 auxiliary condenser and bath are brought into position, and finality of position with 

 respect to the former is obtained by shifting it like the lime on its stand until the 

 field is equally illuminated. The rays should issue parallel from this lens, and to 

 obtain such the point of illumination on the lime should be exactly in the focus 

 of the condenser. To find this it is only needed, as before said, to blow a little 

 tobacco smoke, or burn a little piece of brown paper, in the path of the rays which 

 immediately shows their direction. If they c/^verge the lime is too 7iear the lenses, 

 if they converge the opposite condition is the cause. With some specimens a little 

 convergence improves definition, with others the reverse obtains. 



2. Reflected Light is only used when the upright apparatus is employed. The flat 

 side of the mirror is usually used for high powers, but for the inch and half-inch, 

 and occasionally the sixth, and rarely the eighth and twelfth the curved side is 

 preferable. The mirror should be well silvered and be as tJmi as possible to avoid 

 duplicity of image. 



3. Oblique Light. — In this form of illumination, the light is made to impinge 

 quite obliquely on the condenser and so on the object. It is very obvious then, that 



