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In December, 1839 the newly formed Microscopical Society of London recommended a standard for slide glass sizes (3" x 1", and 3" x 1 1/2") to be used by their members. Prior to that date, there was a wide variation in the sizes being used. These two recommended sizes quickly became the standard for most of the commercial mounters in England, although other sizes (usually smaller) were still produced and used on the Continent (primarily France) for a number of years thereafter. Smaller format slides were also still produced for many years to accompany small "pocket" or drum microscopes, by a number of well known mounters, including Amos Topping. These were often sold in boxed "sets" with the microscope. 

Before that time, throughout the 1700s until circa 1820s, microscopical objects were often mounted for examination using bone, ivory, or hardwood sliders. These objects were either mounted as subjects for Transmitted (through) light examination, or Reflected (top) lighting. 

The transmitted type were made by placing thin sections or specimens between discs of mica (a thin transparent mineral), held in place by brass wire clips in holes drilled through the sliders (see bone sliders #s 1, 2, and 3 below). Objects for use with reflected (top) light were normally mounted uncovered by gluing to paper discs that were then fastened into sliders that had not been drilled completely through, as seen in bone slider #5 (slider #6 is clean and ready for new objects to be mounted) and Ebony Wood sliders (#2,4).

Even as glass become more commonly used for the main slide body during the late 1820s and 1830s, the thin covers placed over the specimens for protection were often still made of mica. The very thin glass required for covers was still quite rare and very expensive. As the thin glass became more readily available and less expensive, mica fell completely out of use, with glass slides and glass covers becoming the norm.
The 2 types of bone sliders shown below are approximately 3 1/2" x 5/8", The J.B. Dancer slide, at 3" x 1" is included as a size reference. The top 3 were made for Transmitted lighting, with the bottom 2 for Reflected or Incident lighting.

Shown above and below, several sizes of larger mahogany wood sliders with the specimen glass in each being approximately 1 1/2". The 3 sliders above are 7" x 2" with 3 specimens in each, all dry mounted between 2 equal thickness pieces of glass. 

The 2 in the bottom image are both 4" x 2", each with a single specimen mounted in Canada Balsam between 2 equal thickness pieces of glass. The mount of Scarlet Ibis Feather is a particularly well prepared, with no entrapped air visible at all. 

A standard size 3" x 1" slide is shown with each group for comparison. Similar sliders are shown in the image below with a typical period microscope that would have been used for viewing them.

Below, a bone slider, unusually with each cell labeled as to content. Most early sliders were just numbered, and came originally with a corresponding hand printed list of specimens. The 3" x 1" Amos Topping mount of Sharks Tooth is shown for size comparison.

Above, 2 early sliders made of Ebony Wood, for examination of the specimens using reflected (top) lighting. They are both 4 1/2" x 5/8" in size (standard 3" x 1" slide shown for comparison). Below are 2 Wood sliders (with mica inserts held by brass wire rings) for transmitted light use. The glass slide is the standard 3" x 1" for size comparison. Interestingly, the larger slider has several of the cells displaying small hand painted images of birds, with the other specimens being a bit of leaf, and a small insect.
Above, 2 very interesting early mounts.  The top one being two large pieces of Flint mounted to the 2" x 3/4" glass slide, and engraved "Flint from Kent with a fish scale" circa 1830s. The bottom slide is a 3" x 1" thin section fossil mount of Echinus Scutella with the label description and signature "Darker" in one hand, and "WR" in another, engraved on the glass. This is a very rare signed example by William Darker, c. early 1840s.  The initials WR refer to W. Rutt (see another example, Human Hair Beard by C. Topping first group under "Unusual" slides section) likely the original owner of the slide. Click here for additional information on W. Rutt

Shown below, a variety of early mounts, circa 1830s. While all are undated, this estimate is based on the sizes and various construction details. All are mounted between two glass slides of similar thickness, some dry mounted, with others mounted in Canada balsam. A Standard size 3" x 1" mount by E. Thum at top left is shown for comparison.

Below, a group of mounts, several retailed by A. Pritchard, likely circa 1830s (others probably early 1840s). Although undated, the address "263 Strand" dates them to between 1836-1839, when he was at that address. Several use the distinctive method of dry mounting between two glass slides of similar thickness, with inner edges beveled, then filled and finished smooth using red sealing wax.  In 1848 Prof. J. Quekett described and referred to this as "Mr. Darker's Method", stating that William H. Darker had perfected and used it since the introduction of the achromatic microscope (early 1830s).  The "American Pine" specimen is what became the "standard" size in 1839, at 3" x 1" 
Many of this last group of slides below, also mostly circa 1830s, have thin covers made of mica over the specimens. These are also all undated, with age estimates based on size and construction details. A standard size 3" x 1" Scales of Podura mount is shown for comparison.