Dust

It is exceedingly difficult to remove from the field certain slight specks which can be seen when looked for. Whenever any specks appear they can at once be located by ascertaining which portion they are in. If on rotating the eyepiece they move, the trouble is there ; if moving the object causes them to traverse the field, the object is accountable for them. If neither of these shows the position, they must be looked for in reflections from the lamp glass, or finally the objective. These specks are not usually very obtrusive, but are observed when the attention is not so much concentrated on the object as on a critical survey for such imperfections.

Working should always be done with the eye close to the eye lens of the eyepiece. If the head is held some little distance above, many fancied defects may appear.

Acquire the habit of working with either eye and keeping open the eye which is not actually employed in observation. Those who find difficulty in doing this should use an eyeshade.

To clean the eyepiece lenses, a soft clean cloth dipped in spirit will remove the dust. The lenses must be carefully wiped dry after such treatment.

When using an oil immersion condenser or Holos immersion parabolois, the use of golden syrup-the ordinary domestic syrup -has been recommended instead of oil. With this, contact is maintained more continuously. It must not be used for Objectives.

How to measure Micro. Objects with the Eyepiece Micrometer and Stage Micrometer.-The stage micrometer is placed on (he stage, and the eyepiece micrometer is put into the eyepiece. This latter is generally divided into parts of a centimeter, but no exact value is needful so long as the lines are equidistant. On focussing the stage micrometer the two sets of lines will appear in the field at once. It is now desirable to ascertain how many divisions of the eyepiece micrometer are included between one of the spaces, that is, l/100th of a millimeter of the stage micrometer. Perhaps it will be found that there will be several lines of the eyepiece micrometer and a fraction in that space, and in order that this fraction may be obviated the draw tube should be slightly pulled out ; this will increase the magnification until a definite number of the lines on the eyepiece micrometer are exactly equal to a division or divisions on the stage micrometer. We will imagine that the number of eyepiece micrometer lines that fill 1/100th of a millimeter of the stage micrometer is 5. The stage micrometer is now removed and the object to be measured replaces it. The lines of the eyepiece micrometer will be seen in the field, and bearing in mind that five of these lines equal l/100th of a millimeter, any part of the object can at once be measured.

It must be remembered, however, that with every Objective and at every tube length an estimation of the value of the eyepiece micrometer is necessary.

Large Lenses And High Magnification

To those who will put the matter on paper thoughtfully, it will immediately become apparent that a combination of large lenses and high magnification is impossible.

It is generally known that magnifying power depends on the focus of the lens, and that the focus of the lens depends on its radius.

Bearing in mind that the focus is twice the radius (approximately), it will be seen that a lens with its maximum curvature, viz., a hemisphere, could only have a focus equal to its diameter.

Suppose now we assume that the lens is lin. in diameter, and has the maximum radius of lin., the utmost magnification that could be obtained with it would be 10 diameters. Increased magnification would of necessity mean a smaller size of lens, because magnifying power depends on focus, a lens lin. diameter having a nominal power of 10, one of 1/2in. 20 ;diameters, and so on.

It so frequently happens that a lens of 3 inches or even 4 inches diameter of high power is specified. The former could easily magnify about 31/3 diameters and the latter 21/2 ;diameters, and no more-not very " high " magnification-and the spherical aberration or distortion would make it almost valueless.

Combinations of lenses enable a variation to be made in favour of somewhat larger diameter than is the case with a single lens, but in the main the lines mentioned may be taken as definite, and if high magnification is wanted, it cannot be in conjunction with lenses of large diameter.

The question of spherical aberration, distortion, etc., is left out of the above consideration, but they obviously play a very important part, especially with large lenses.

Generally speaking, a lens should be expected to be not larger in diameter than half its focus. A 1/2in. lens of 1in. focus, and a 1in. lens of 2in. focus are good working conditions.

Microscope Objectives. However perfectly a microscope stand may he constructed, it is useless without its optical equipment.

On the lenses that are used, depends the quality of the image that is seen. The selection, therefore, of suitable Objectives for the class of work to be done is of the utmost importance.

This is fully recognised by manufacturers, and the high degree of perfection that has been attained in modern microscope objectives reflects their enterprise and scientific skill.

The large number and variety of the types of optical glass offer a wide choice in the computations of formulae, and the successful efforts of British optical glass manufacturers to meet the needs of opticians have resulted in the supply of glass which is unsurpassed for the purpose.

There is no need to consider foreign made optical glass or lenses for first class work; British makers are easily first, all and any statements to the contrary being incorrect.