A mechanical counting device locatable within a main bore of a downhole tool which includes a plurality of annular recesses arranged longitudinally along the main bore, the mechanical counting device for use in actuating the tool to open one or more fluid ports which are transverse to the main bore, the mechanical counting device comprising: linear indexing means comprising a collet member which comprises: a tubular member having a bore which is sized such that an object may pass through the tubular member; and a first set of fingers and a second set of fingers which are longitudinally spaced apart from the first set of fingers, wherein each finger (26, 28) includes a protrusion provided at the end thereof and each finger (26, 28) is moveable between a first position in which the protrusion is outwith the bore of the tubular member and a second position in which the protrusion is within the bore of the tubular member and contactable by an object passing through the tubular member; wherein the collet member is locatable within the main bore of the downhole tool such that the protrusion of at least one finger (26, 28) is engaged with a recess when said at least one finger (26, 28) is at the first position and not engaged with a recess when said at least one finger (26, 28) is at the second position, and wherein the collet member and the recesses are configured such that, when the protrusions of the first set of fingers are engaged with a recess, the protrusions of the second set of fingers are not engaged with a recess, and when the protrusions of the second set of fingers are engaged with a recess, the protrusions of the first set of fingers are not engaged with a recess; wherein the collet member is configured to be linearly progressed along the main bore of the downhole tool by a predetermined distance in response to receiving an object transported through the collet member and sequentially engaging the protrusions of the first and second sets of fingers (26, 28), until reaching an actuation site of the tool whereupon the tool is actuated, wherein the mechanical counting device is locatable at a plurality of different predetermined positions within the main bore of the downhole tool linear indexing means comprising a collet member which comprises: a tubular member having a bore which is sized such that an object may pass through the tubular member; and a first set of fingers and a second set of fingers which are longitudinally spaced apart from the first set of fingers, wherein each finger (26, 28) includes a protrusion provided at the end thereof and each finger (26, 28) is moveable between a first position in which the protrusion is outwith the bore of the tubular member and a second position in which the protrusion is within the bore of the tubular member and contactable by an object passing through the tubular member; a tubular member having a bore which is sized such that an object may pass through the tubular member; and a first set of fingers and a second set of fingers which are longitudinally spaced apart from the first set of fingers, wherein each finger (26, 28) includes a protrusion provided at the end thereof and each finger (26, 28) is moveable between a first position in which the protrusion is outwith the bore of the tubular member and a second position in which the protrusion is within the bore of the tubular member and contactable by an object passing through the tubular member; wherein the collet member is locatable within the main bore of the downhole tool such that the protrusion of at least one finger (26, 28) is engaged with a recess when said at least one finger (26, 28) is at the first position and not engaged with a recess when said at least one finger (26, 28) is at the second position, and wherein the collet member and the recesses are configured such that, when the protrusions of the first set of fingers are engaged with a recess, the protrusions of the second set of fingers are not engaged with a recess, and when the protrusions of the second set of fingers are engaged with a recess, the protrusions of the first set of fingers are not engaged with a recess; wherein the collet member is configured to be linearly progressed along the main bore of the downhole tool by a predetermined distance in response to receiving an object transported through the collet member and sequentially engaging the protrusions of the first and second sets of fingers (26, 28), until reaching an actuation site of the tool whereupon the tool is actuated, wherein the mechanical counting device is locatable at a plurality of different predetermined positions within the main bore of the downhole tool