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General information on microscope objectives and eye pieces.
Microscope Objectives and Eye-Pieces
Although considerable magnifying power may be attained by the use of two single lenses arranged in a compound form, there is no advantage in it, from the fact that the faults in the lenses are correspondingly magnified, and these are so considerable that they destroy what it is the purpose of the microscope to give a distinct image. Classes of Microscope Objectives Objectives may be divided into two classes, dry and immersion. Dry Objectives - Microscopy Dry objectives have no intervening medium except air exists between the cover and objective. Immersion Objectives - Microscopy In immersion or "wet" objectives a fluid is used to connect the upper surface of the cover to the front surface of the objective. The use of immersion fluid has several advantages, the first of which is that the objective may be made to give better performance, as will be explained later on ; the second is that more light will be transmitted, as there is less loss of it by refraction. It should be understood however that no advantage will be gained by using immersion fluid with a dry objective. It does not increase its effectiveness one particle, on the contrary it detracts from its quality. When it is stated that an immersion objective has a greater capacity, it is with the understanding that it is so constructed as to give this result. While many immersion objectives are constructed to work both as dry and immersion, such a plan cannot be said to be advantageous. Such objectives may be made to work well in one direction and be of indifferent quality in the other, or may be of medium grade both ways. There is no question that the best plan .is to have each objective selected with a view to a specific purpose and use it for this purpose only. There are two fluids in general use at the present time, water and homogeneous fluid. The latter expression means of the same kind, and refers to the fact that the fluid has about the same refractive and dispersive power as glass, so that when this fluid fills up the space between the two surfaces of glass, a ray of light passes through the three mediums as if they were one body. The two large classes of dry and immersion objectives may again be subdivided into two classes objectives for long and short standard tube. As followed by some firms at present and what it is hoped will become a universal custom in time, each objective is marked for the tube-length for which it is corrected and with which it is assumed it will accomplish the best results. Objectives are sometimes called powers, and in this sense are divided into three classes : low, medium and high powers. Dr. Carpenter classifies them as follows: Low powered objectives are: 3 inch, 2 inch, 1 1/2 inch, 1 inch, 2/3 and 3/4 of an inch. Medium powered objectives: 4/10 of an inch, 1/2 inch, 1/4 inch, 1/5 inch ; High powered objectives are: 1/6 inch, 1/8 inch, 1/10th of an inch, 1/12 inch, 1/16 inch, 1/20 inch, 1/25 inch. Objectives - Microscope Optics As the objective is the most important of the two optical parts, it follows that this must be as free from faults as possible and all that human ingenuity and skill can devise is utilized to attain this end. The advance in the perfection of the objective has been step by step and each era was at the time considered by many authorities the limit to further improvement. Each advance was signalized by a marked opposition and disbelief of its possibility. It is therefore of inestimable creditto the pioneer objective-makers, and notably among these two Americans, who by quiet but stubborn application disproved previous claims and opened the way to further improvements. A theoretical limit has been fixed on the capacity of the microscope, which according to our present knowledge can not even be reached. While the introduction of water immersion made it possible to obtain higher optical results than with the dry objectives, as will be explained later on, the homogeneous immersion offers still greater possibilities in this direction, and the advantages are so pronounced that the former are gradually coming into disuse, although for certain kinds of work they will be preferred and used by many persons. At present homogeneous fluid is made of either thickened glycerine or cedar oil, and great care is required in keeping the front of objectives and cover glasses properly cleaned, in which respect water has the decided advantage. It might be stated that such high power as 1/25 th and1/20 th were very rarely constructed, and the 1/16 th may be considered the maximum, while the 1/12 th is that most ordinarily used. This power will give all the optical advantages, while higher powers involve so many mechanical difficulties as to increase the cost of production very considerably, and as a rule detract from the optical qualities. A modern objective of the highest capacity may be considered a work of art, and there are a few productions of the human hand which exact so much untiring application, ingenuity and skill. Objective Systems An objective is said to consist of systems which may vary in number from one to four and five ; two and three are however mainly in use. They are the individual portions consisting of one, two or three lenses, which when more than one, are cemented together and make up the objective. An achromatic sinP gle system may consist of two or three lenses, and a three or four system objective may consist of as many as seven or eight lenses. The systems are called in their A order : anterior or front, middle and posterior. When one consists of two lenses it is called a doublet, when of three lenses a triplet. A is the anterior, M the middle and P the posterior systems ; thus also A is a single system, M a double and P a triple one. The various features which must be considered as determining the quality of an objective are : angular aperture, achromatism, resolving power, flatness of field, penetration, working distance and magnifying power. Although these attributes may be considered separately, some of them go hand in hand. The presence or extent of one necessarily involves or precludes another.
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