The microscope : and its application to vegetable anatomy and physiology / by Dr. Hermann Schacht ; edited by Frederick Currey, M. A.

Hermann Schacht

Date:: 1855

Credit: The microscope : and its application to vegetable anatomy and physiology / by Dr. Hermann Schacht ; edited by Frederick Currey, M. A. Source: Wellcome Collection.

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$i-apid progi-ess lias been made of late years. Mr. Ross’ aeliro- matic combinations of l-8tli of an inch focal length, are made to transmit angular pencils of 175°. In connection with the object-glass it is necessary to mention the adjustment which is made for viewing covered objects. When an object-glass has been corrected for spherical aben-a- tion, so as to attbrd a perfect image of an uncovered object, it is foimd that if the object is covered with a piece of thin glass, tlie correction of the aberration is so far interfered with as to lender the performance of the object-glass defective. This effect of the thin glass is remedied by an arrangement by which the distance of the anterior lens of the combination from the other two is rendered capable of being varied within certain limits sufficient to correct the error in the abeiTation caused by the tliiu glass. The adjustment is effected by turning a screw collar at the ujiper end of the object-glass; and ]\Ir. Ross, by v.diom the aiu’angement was invented, gives the following dii’ections with regard to it:— “ When an achx'omatic object-glass for a microscope has its aberrations corrected for viewing an micovered object, the cor- rection will be nearly the same, whether the object is seen by the light reflected from its sui’face as an opaque, or by its inter- cepting transmitted light as a transpai-ent one, if these objects ai-e properly prepai-ed and illuminated; but if it be necessary to cover the object with glass or tide, or to immei’se it in a fluid, the abeiTation caused by the refractive and dispei’sive power of the interposed medium deteriorates the performance of the ob- ject-glass. The adjustment which is given to olject-glasses of high magnifying power, and ti'ansmitting lai'ge angular jiencils of light, is for the purpose of compensating the aberration re- siUting from the various states in which an object may be placed. To effect this there are two lines on the external part of the object-ghiss; against the upper line is engi’aved un- Govered, and against the lower, covered; there is also a small square piece of brass, or tongue, screwed into a morticed hole, with a single line upon it, as shewn in fig. 25, Immediately above the lines is a projecting milled edge, which may be moved independently of the other part of the object-glass, giving$