Microscope Substage Condensers

-A high degree of excellence in the illuminating apparatus of a microscope cannot be too strongly insisted upon. With low powers the light reflected directly from a mirror is generally sufficient, but not so with the high powers. Here it is not only necessary to obtain more light, but it is imperative that such light should be under perfect control, and that the quality of the system producing the illuminating cone should be as carefully considered as the objective itself. In short, the condenser is a necessary adjunct : not a mere refinement. The better the quality of objective used, the more perfect must be the substage condenser, if the full virtue of the former is to be exhibited. A well-corrected objective demands a condenser having an aplanatic aperture which is proportionate to its own. A good objective under favourable conditions will bear a solid cone of illumination of three-fourths its numerical aperture : the condenser should be selected accordingly.

Fig. 122.

Fig. 122 shows the general construction and design of a universal type of substage condenser; it has a triple back lens for corrective purposes, and the whole of the aperture is aplanatic.

Fig. 123.

Fig. 123 illustrates an oil immersion substage condenser. The elimination of spherical aberration is indicated by the large aplanatic aperture. The numerical aperture of this condenser varies according to the thickness of slip through which it has to work, normally it is made to work through slips up to 1-6 m/m thick and then has a N.A. of about 1-34. If made (to order) to work through slips not exceeding 1-3 m/m, its numerical aperture can be brought up to 1 -40. On the other hand, if greater working distance is demanded, the N.A. falls proportionately to 1-30.

The Substage Condenser

In order, however, to develop the full capacity of the modern microscope, it is essential that a substage condenser be employed. This, in the case of a student's microscope, fits into the underfitting 9 in Fig. 120 ; or in the case of a microscope having a compound substage into the fitting 25 in Fig. 121. In the former instance the tube is already set central before the microscope leaves the works, but latitude is allowed for resetting by slackening the screws by which the tube flange is attached to the bracket, which latter has slots to permit of movement. Generally speaking, however, this should only be done by experienced workers.

Important Notice

The condenser is pushed upwards in tube 9 (which is of the Royal Microscopical Society's standard gauge) until the upper surface of the top lens comes flush with the stage surface. This can be exactly arranged by placing an object on the surface of the stage and holding it firmly and adjusting the condenser, until the desired position is attained.

If the microscope has a substage, as in the illustration in Fig. 121, the condenser is inserted in it and the rack-work employed to carry it up to about level with the surface of the stage. It is then necessary to see that it is central with the objective. It should be explained that differences of centres occur with objectives, especially if they are by different makers, and the only way to secure the maximum effect from a condenser is by carefully centring it in the sub-stage.

To Centre The Condenser

(a) When using low-power objectives, say lin. or fin.

There are two ways in which this can be done :-

1. By putting a mark on the upper surface of the top lens of the condenser.

2. By using the Iris diaphragm at the back of the optical part for the purpose.

Process 1 is the best with the Abbe Illuminator. A very small spot of ink should be set as nearly on the centre of the top lens as possible. This should then be viewed through the microscope as if it were an object, and the centring screws 26 turned until it occupies the centre of the field. The spot can then be wiped off.

With achromatic condensers a lin. or fin. objective is employed ; light is thrown through the condenser and the Irus diaphragm closed down to its smallest aperture. The objective should then be very slightly racked upward, it being necessary to focus and centre the aerial image formed by the dis-phragm through the lens of the condenser. This generally lies in a plane about 1/8 in. above the surface of the condenser lens. This is not easy to do at first, but with a little practice facility will be attained.

When the centring has been accomplished, the low-power objective is removed and the high-power interchanged.

If, at any time, it is desired to check the centring of the condenser when using high-power objectives, the eyepiece 10 can be removed and the back lens of the objective examined by looking down the tube of the microscope. If then the Iris diaphragm of the condenser be gradually open and closed, its centrality can be verified.

To Focus The Condenser

The condenser requires not only to be centred, but also to be focussed in the plane of the object that is under examination, and this should be as accuratelv done as with the objective. The larger the aperture and the higher the power of the condenser, the greater is the accuracy of manipulation required.

As a general guide, however, it may be stated that the Abbe Illuminator, Universal Condenser, and similar condensers are about in focus when the front lens is approximately 1 /25th inch from the under side of the object, this distance varying with the thickness of the object slide and the distance of the illuminant.

The method of focussing is as follows :-

Using the plane mirror and the edge of a lamp flame as the source of light, the lamp is set about 4 inches from the mirror. The light is then directed through the condenser, the upper surface of the lens of which is to be in contact with the under side of the object.